Chapter 9 and 10 Biology

Carbohydrates, fats, and proteins are

a store of XX XX for cellular work

Carbohydrates, fats, and proteins store XX XX. During aerobic XX XX, cells slowly release that energy and use it to make XX.

Instead of “burning” glucose all at once, cells break food down XX by XX so they can capture the energy efficiently.

During this process:

• XX is removed from food gradually.

• Hydrogen is separated into:

  • XX ions (XX)

  • XX (XX)

• The electrons travel through the XX XX XXX (XX) in the XX XX.

• Energy from the electrons is used to pump XX into the XX XX

• This creates a XX XX XX, which is stored potential energy.

• H⁺ then flows back into the XX XX through XX XX

• ATP synthase works kind of like a XX, using that flow to make XX.

The dam picture is basically the analogy:

Water stored behind dam = X stored in XX XX
Water flowing through turbine = X flowing throughXX XX
Electricity produced = XX XX

And because this is aerobic cellular respiration, XX is required. Oxygen ultimately accepts the XX at the end of the ETC.

Carbohydrates, fats, and proteins are

a store of potential energy for cellular work

Carbohydrates, fats, and proteins store potential energy. During aerobic cellular respiration, cells slowly release that energy and use it to make ATP.

Instead of “burning” glucose all at once, cells break food down step by step so they can capture the energy efficiently.

During this process:

• Hydrogen is removed from food gradually.

• Hydrogen is separated into:

  • H⁺ ions (protons)

  • electrons (e⁻)

• The electrons travel through the electron transport chain (ETC) in the inner mitochondrial membrane.

• Energy from the electrons is used to pump H⁺ into the intermembrane space.

• This creates a hydrogen ion gradient, which is stored potential energy.

• H⁺ then flows back into the mitochondrial matrix through ATP synthase.

• ATP synthase works kind of like a turbine, using that flow to make ATP.

The dam picture is basically the analogy:

Water stored behind dam = H⁺ stored in intermembrane space
Water flowing through turbine = H⁺ flowing through ATP synthase
Electricity produced = ATP produced

And because this is aerobic cellular respiration, oxygen is required. Oxygen ultimately accepts the electrons at the end of the ETC.

The 4 stages of aerobic cellular respiration are:

1. Glycolysis

   • Happens in the XX

   • 1 XX (6 XX) is split into X pyruvate (3 XX each)

   • Produces XX XX

   • Also removes some XX/electrons that are carried by XX

2. Acetyl-CoA formation

   • Pyruvate enters the XX

   • Each pyruvate loses 1 XX as XX

   • The remaining X-carbon molecule becomes XX

   • More XX/electrons are removed and carried by NADH

3. Krebs Cycle

   • Happens in the XX XX

   • XX is broken down further

   • More XX is released

   • Produces a small amount of XXX

   • Removes most of the remaining XX/electrons

   • These are carried mainly by XX and XX

4. Electron Transport Chain + Chemiosmosis

   • Happens in the XX XX XX

   • XX and XX bring electrons to the ETC

   • Electron energy pumps X⁺ into the XX XX

   • H⁺ flows back through XX XX

   • This makes most of the XX

   • XX is the final electron acceptor and combines with H⁺ to form water

What your professor means with the hydrogens

Glucose is C₆H₁₂O₆, so it contains XX hydrogens.

Your professor is basically tracking how those hydrogens are gradually removed during respiration:

Glucose → glycolysis → pyruvate → acetyl-CoA → Krebs cycle

The hydrogens/electrons are not just thrown away. They are picked up by XX andXX, becoming XX and XX, and then delivered to the XX

Carbon tracking

This is also important:

1 glucose = X carbons

After glycolysis:
2 pyruvate × 3 carbons = X carbons

During acetyl-CoA formation:
X CO₂ are released total

During Krebs cycle:
X more CO₂ are released

So overall:

X carbons in glucose → X CO₂

And the main idea is:

If oxygen is present → XX enters the mitochondria → XX XX + XX occur → lots of XX is produced.

If oxygen is not available, the cell cannot keep the ETC running normally, so it may use XX instead.

The 4 stages of aerobic cellular respiration are:

1. Glycolysis

   • Happens in the cytoplasm

   • 1 glucose (6 carbons) is split into 2 pyruvate (3 carbons each)

   • Produces 2 ATP

   • Also removes some hydrogen/electrons that are carried by NADH

2. Acetyl-CoA formation

   • Pyruvate enters the mitochondrion

   • Each pyruvate loses 1 carbon as CO₂

   • The remaining 2-carbon molecule becomes acetyl-CoA

   • More hydrogen/electrons are removed and carried by NADH

3. Krebs Cycle

   • Happens in the mitochondrial matrix

   • Acetyl-CoA is broken down further

   • More CO₂ is released

   • Produces a small amount of ATP

   • Removes most of the remaining hydrogen/electrons

   • These are carried mainly by NADH and FADH₂

4. Electron Transport Chain + Chemiosmosis

   • Happens in the inner mitochondrial membrane

   • NADH and FADH₂ bring electrons to the ETC

   • Electron energy pumps H⁺ into the intermembrane space

   • H⁺ flows back through ATP synthase

   • This makes most of the ATP

   • Oxygen is the final electron acceptor and combines with H⁺ to form water

What your professor means with the hydrogens

Glucose is C₆H₁₂O₆, so it contains 12 hydrogens.

Your professor is basically tracking how those hydrogens are gradually removed during respiration:

Glucose → glycolysis → pyruvate → acetyl-CoA → Krebs cycle

The hydrogens/electrons are not just thrown away. They are picked up by NAD⁺ and FAD, becoming NADH and FADH₂, and then delivered to the ETC.

Carbon tracking

This is also important:

1 glucose = 6 carbons

After glycolysis:
2 pyruvate × 3 carbons = 6 carbons

During acetyl-CoA formation:
2 CO₂ are released total

During Krebs cycle:
4 more CO₂ are released

So overall:

6 carbons in glucose → 6 CO₂

And the main idea is:

If oxygen is present → pyruvate enters the mitochondria → Krebs cycle + ETC occur → lots of ATP is produced.

If oxygen is not available, the cell cannot keep the ETC running normally, so it may use fermentation instead.

How does energy flow between processes?

Photosynthesis:

XX

(bacterial, plant

and algal cells)

CO2 is XX and

H2O is XX

to make sugar.

Result: XX

for cellular work!

Electron donor = XX agent

Electron acceptor =XXagent

Aerobic respiration:

Mitochondria (and

aerobic bacteria)

Glucose is XX to

form XX and XX is

reduced to form water.

This is done by a series

of XX XX

(oxidation-reductions)

“LEO-GER” :

“Lose electrons XX”

“Gain Electrons XX”

Aerobic cellular respiration

Overall:

Glucose + O₂ → CO₂ + H₂O + energy (ATP)

During this process:

• Glucose is XX

  • It loses sXX and XX

  • Its carbons eventually become XX

• Oxygen is XX

  • It gains XX andXX

  • It eventually becomesXX

Remember:

LEO-GER

• Lose Electrons = XX

• Gain Electrons = XX

So:

Glucose → loses XX/X → XX
O₂ → XX electrons/X → XX

The cell does not transfer all the electrons/hydrogens at once. It removes them in many XX XX, which lets the cell capture the released energy and use it to make XX.

Photosynthesis is basically the reverse overall process

CO₂ + H₂O + light energy → glucose + O₂

During photosynthesis:

• CO₂ is XX to help make XX

• H₂O is XX, producing XX

• XX provides the energy needed

The cycle between the two

Photosynthesis:
XX + XX → XX + XX

Cellular respiration:
XX +XX → XX+ XX + XX

So the XX of one process become the XX of the other.

Important correction from the transcript

Glucose is XX to CO₂.

And:

Oxygen isXX to H₂O.

A really good exam shortcut is:

Respiration: glucose XX, XX reduced
Photosynthesis: XX reduced, XX oxidized

How does energy flow between processes?

Photosynthesis:

Chloroplasts

(bacterial, plant

and algal cells)

CO2 is reduced and

H2O is oxidized

to make sugar.

Result: ATP

for cellular work!

Electron donor = Reducing agent

Electron acceptor = Oxidizing agent

Aerobic respiration:

Mitochondria (and

aerobic bacteria)

Glucose is oxidized to

form CO2 and oxygen is

reduced to form water.

This is done by a series

of Redox Reactions

(oxidation-reductions)

“LEO-GER” :

“Lose electrons Oxidized”

“Gain Electrons Reduced”

Aerobic cellular respiration

Overall:

Glucose + O₂ → CO₂ + H₂O + energy (ATP)

During this process:

• Glucose is oxidized

  • It loses electrons and hydrogen

  • Its carbons eventually become CO₂

• Oxygen is reduced

  • It gains electrons and hydrogen

  • It eventually becomes H₂O

Remember:

LEO-GER

• Lose Electrons = Oxidized

• Gain Electrons = Reduced

So:

Glucose → loses electrons/H → oxidized
O₂ → gains electrons/H → reduced

The cell does not transfer all the electrons/hydrogens at once. It removes them in many small steps, which lets the cell capture the released energy and use it to make ATP.

Photosynthesis is basically the reverse overall process

CO₂ + H₂O + light energy → glucose + O₂

During photosynthesis:

• CO₂ is reduced to help make glucose

• H₂O is oxidized, producing O₂

• Light provides the energy needed

The cycle between the two

Photosynthesis:
CO₂ + H₂O → glucose + O₂

Cellular respiration:
glucose + O₂ → CO₂ + H₂O + ATP

So the products of one process become the reactants of the other.

Important correction from the transcript

Your professor says at one point:

“Glucose is reduced to CO₂”

That wording is incorrect.

It should be:

Glucose is OXIDIZED to CO₂.

And:

Oxygen is REDUCED to H₂O.

A really good exam shortcut is:

Respiration: glucose oxidized, O₂ reduced
Photosynthesis: CO₂ reduced, H₂O oxidized

Redox Reactions in Cellular Respiration

A redox reaction is a XX XX where electrons are XX between molecules. XX often moves along with those electrons.

Oxidation vs. Reduction

• Oxidation = XX electrons

• Reduction = XX electrons

• They always occur together: if one molecule loses electrons, another must gain them.

LEO-GER

• LEO = XX Electrons → XX

• GER =XX Electrons → XX

In Cellular Respiration

Overall:

C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + energy

Glucose

Glucose:

• loses XX

• loses XX

• becomes XX

• is XX

So:

Glucose → CO₂ = XX

Glucose is also the:

XX XX

Because it donates electrons, glucose is called the:

XX XX

Important: the reducing agent itself gets oxidized.

Oxygen

Oxygen:

• gains XX

• gains XX

• becomes XX

• is XX

So:

O₂ → H₂O = reduction

Oxygen is also the:

XX XX

Because it accepts electrons, oxygen is called the:

XX XX

Important: the oxidizing agent itself gets reduced.

Why Oxygen Matters

Oxygen is necessary for XX cellular respiration.

If oxygen is present:

• it accepts the XX/hydrogens removed from glucose

• oxygen becomes XX

• water is XX

• aerobic respiration can continue

If oxygen is not present:

• this aerobic redox pathway cannot continue normally

• the electron transport chain cannot keep operating normally

Why You Need Both Oxidation and Reduction

You cannot have oxidation by itself.

If glucose loses electrons, something else must XX them.

So:

• glucose is oxidized

• oxygen accepts those electrons and is reduced

That is why oxidation and reduction are always linked.

Why Redox Reactions Are Important for Energy

The cell does not remove all the electrons and hydrogens from glucose at once.

It removes them gradually in many XX

This allows the cell to:

• release energy XX

• capture that XX

• use it to make XX

Key relationship to memorize

Glucose

• loses XX

• XX

• electron XX

• XX XX

Oxygen

• gains XX

• XX

• electron XX

• XX agent

Best shortcut

Glucose gives electrons → oxidized → reducing agent

Oxygen receives electrons → reduced → oxidizing agent

Redox Reactions in Cellular Respiration

A redox reaction is a chemical reaction where electrons are transferred between molecules. Hydrogen often moves along with those electrons.

Oxidation vs. Reduction

• Oxidation = losing electrons

• Reduction = gaining electrons

• They always occur together: if one molecule loses electrons, another must gain them.

LEO-GER

• LEO = Lose Electrons → Oxidized

• GER = Gain Electrons → Reduced

In Cellular Respiration

Overall:

C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + energy

Glucose

Glucose:

• loses electrons

• loses hydrogen

• becomes CO₂

• is oxidized

So:

Glucose → CO₂ = oxidation

Glucose is also the:

electron donor

Because it donates electrons, glucose is called the:

reducing agent

Important: the reducing agent itself gets oxidized.

Oxygen

Oxygen:

• gains electrons

• gains hydrogen

• becomes H₂O

• is reduced

So:

O₂ → H₂O = reduction

Oxygen is also the:

electron acceptor

Because it accepts electrons, oxygen is called the:

oxidizing agent

Important: the oxidizing agent itself gets reduced.

Why Oxygen Matters

Oxygen is necessary for aerobic cellular respiration.

If oxygen is present:

• it accepts the electrons/hydrogens removed from glucose

• oxygen becomes reduced

• water is formed

• aerobic respiration can continue

If oxygen is not present:

• this aerobic redox pathway cannot continue normally

• the electron transport chain cannot keep operating normally

Why You Need Both Oxidation and Reduction

You cannot have oxidation by itself.

If glucose loses electrons, something else must accept them.

So:

• glucose is oxidized

• oxygen accepts those electrons and is reduced

That is why oxidation and reduction are always linked.

Why Redox Reactions Are Important for Energy

The cell does not remove all the electrons and hydrogens from glucose at once.

It removes them gradually in many steps.

This allows the cell to:

• release energy slowly

• capture that energy

• use it to make ATP

Key relationship to memorize

Glucose

• loses electrons

• oxidized

• electron donor

• reducing agent

Oxygen

• gains electrons

• reduced

• electron acceptor

• oxidizing agent

Best shortcut

Glucose gives electrons → oxidized → reducing agent

Oxygen receives electrons → reduced → oxidizing agent

Mitochondrion: Structure & FunctionMain Function

The mitochondrion is a XX XX-bound organelle found in XX cells.

Its main roles are:

• XX utilization

• XX production

• Carrying out aerobic XX XX

Aerobic Bacteria vs. Eukaryotic Cells

Some bacteria are XX, meaning they can use oxygen for XX XX.

However:

• Bacteria do not have XX

• They perform these reactions using their XX XXFor eukaryotic cells, aerobic cellular respiration mainly occurs in the XX

Structure of the Mitochondrion1. Outer Membrane

The XX XX forms the outside boundary of the mitochondrion.

It is more similar to a typical cell membrane.

2. Intermembrane Space

The intermembrane space is the space between the XX and XX mitochondrial membranes.

This area is important because:

• XX XX build up here

• This creates a XX XX

• The proton gradient is later used to make XX

3. Inner Membrane

The inner mitochondrial membrane is very important for XX XX XX

It is:

• highly XX

• more XX-like in nature

• the location of the XX XX XX

The inner membrane also contains:

• XX XX XX

• XX XX

4. Cristae

The folds of the inner membrane are called:

XX

The inner membrane is highly folded to:

increase XX XX

More surface area allows for:

• more XX XX XX

• more XX XX

• more XX XX

Important:

The electron transport chain occurs on the XX mitochondrial membrane/cristae.

5. Matrix

The matrix is the XX XX interior of the mitochondrion.

Your professor described it as the:

liquid part of the mitochondrion

Important:

The XX XX occurs in the mitochondrial matrix.

So remember:

XX→ Krebs cycle

Inner membrane/cristae →XX XX XX

Mitochondrial DNA and Ribosomes

Mitochondria contain:

• their own XX XX

• XX XX

• a small XX

These features are another reason XX have similarities to bacteria.

Maternal Inheritance

Your professor also mentioned that:

Your mitochondria come from your XX

Mitochondrial DNA is generally inherited through the egg from the mother.

Important Locations to Know

• Outer membrane → XX XX

• Intermembrane space → XX accumulates here

• Inner membrane →XX and XX XX

• Cristae → folds of XX XX that increase XX XX

• Matrix → XX XX

• Mitochondrial DNA → mitochondrion's own small XX

• Free ribosomes → help make some mitochondrial XX

Mitochondrion: Structure & FunctionMain Function

The mitochondrion is a double-membrane-bound organelle found in eukaryotic cells.

Its main roles are:

• O₂ utilization

• ATP production

• Carrying out aerobic cellular respiration

Aerobic Bacteria vs. Eukaryotic Cells

Some bacteria are aerobic, meaning they can use oxygen for cellular respiration.

However:

• Bacteria do not have mitochondria

• They perform these reactions using their cell membrane

For eukaryotic cells, aerobic cellular respiration mainly occurs in the mitochondria.

Structure of the Mitochondrion1. Outer Membrane

The outer membrane forms the outside boundary of the mitochondrion.

It is more similar to a typical cell membrane.

2. Intermembrane Space

The intermembrane space is the space between the outer and inner mitochondrial membranes.

This area is important because:

• H⁺ ions build up here

• This creates a proton gradient

• The proton gradient is later used to make ATP

3. Inner Membrane

The inner mitochondrial membrane is very important for cellular respiration.

It is:

• highly folded

• more bacterial-like in nature

• the location of the electron transport chain

The inner membrane also contains:

• electron transport proteins

• ATP synthase

4. Cristae

The folds of the inner membrane are called:

✅ Cristae

The inner membrane is highly folded to:

increase surface area

More surface area allows for:

• more electron transport chains

• more ATP synthase

• more ATP production

Important:

✅ The electron transport chain occurs on the inner mitochondrial membrane/cristae.

5. Matrix

The matrix is the fluid-filled interior of the mitochondrion.

Your professor described it as the:

liquid part of the mitochondrion

Important:

✅ The Krebs cycle occurs in the mitochondrial matrix.

So remember:

Matrix → Krebs cycle

Inner membrane/cristae → Electron transport chain

Mitochondrial DNA and Ribosomes

Mitochondria contain:

• their own mitochondrial DNA

• free ribosomes

• a small genome

These features are another reason mitochondria have similarities to bacteria.

Maternal Inheritance

Your professor also mentioned that:

Your mitochondria come from your mother.

Mitochondrial DNA is generally inherited through the egg from the mother.

Important Locations to Know

• Outer membrane → outside boundary

• Intermembrane space → H⁺ accumulates here

• Inner membrane → ETC and ATP synthase

• Cristae → folds of inner membrane that increase surface area

• Matrix → Krebs cycle

• Mitochondrial DNA → mitochondrion's own small genome

• Free ribosomes → help make some mitochondrial proteins

NAD⁺ and NADH: Electron ShuttlesWhat is NAD⁺?

NAD⁺ = XX XX XX

NAD⁺ is an XX XX used during XX XX

Its main job is to:

• pick up XX and XX removed from food

• carry them to the XX XX XX

• return and pick up more XX again

We make NAD⁺ using vitamin B3 (niacin) from our diet.

Why is it called a dinucleotide?

NAD⁺ is made of two XX, which is why it is called a dinucleotide.

One important part contains the nitrogenous base:

nicotinamide

The nicotinamide part is where electrons and hydrogen are XX

How NAD⁺ Works During Cellular Respiration

As food is broken down, XX atoms are removed from it.

They are generally removed:

X hydrogen atoms at a time

The enzymes that remove these hydrogens are called:

XX enzymes

Dehydrogenase enzymes remove XX from food molecules.

What happens to the 2 hydrogen atoms?

Two hydrogen atoms contain:

2 H⁺ + 2 e⁻

NAD⁺ accepts:

• X electrons

• X proton (H⁺)

This changes XX into:

XX

The other proton is released as:

X

So the simplified reaction is:

XX+ XX → XX + XX

Another way to think about it:

NAD⁺ picks up:

• one whole XX atom = H⁺ + e⁻

• one additional electron from the second hydrogen

So NADH carries:

2 electrons + 1 H⁺

The other H⁺ remains separate.

NAD⁺ vs. NADH

• XX form

• has not picked up the XX yet

• acts like an empty electron shuttle

• XX charged

NADH

• XX form

• has gainedXX

• carries XX-XX electrons

• acts like a loaded electron shuttle

Remember:

GER = Gain XX → XX

So:

NAD⁺ gains XX→ NADH

That means NAD⁺ is reduced when it becomes NADH.

What Happens to NADH?

NADH carries its electrons to the:

XX electron transport chain

At the ETC:

• XX gives up its electrons

• the energy from those electrons helps create theXX gradient

• that gradient is used to make XX

After NADH gives away its electrons, it becomes:

XX again

Then NAD⁺ can go back and pick up more electrons.

So the cycle is:

NAD⁺ → picks up electrons → NADH → delivers electrons to ETC → NAD⁺

Oxidation and Reduction

When:

NAD⁺ → NADH

NAD⁺ gains electrons, so it is:

XX

When:

NADH → NAD⁺

NADH loses XX, so it is:

XX

This follows:

LEO-GER

• Lose Electrons = Oxidized

• Gain Electrons = Reduced

Structural Change Your Professor MentionedNAD⁺

• XX form

• nicotinamide ring is XX XX

• has 3 XX XX in the ring

NADH

• XX form

• accepts 2 XX and 1 proton

• is no longer XX XX

• has 2 double bonds in the ring

The main thing to remember is:

NAD⁺ = oxidized form

NADH = reduced form

Other Electron Shuttles

Two similar electron carriers are:

• FAD → used in XX XXX

• NADPH → used mainly in XX

Important Summary

NAD⁺

• XX form

• empty XX

• accepts XX and XX

• becomes NADH

NADH

• XX XX

• loaded XX

• carries electrons to the XX

• helps with XX production

Dehydrogenase enzymes

• remove XXX from food

Best shortcut

NAD⁺ picks up XX → XX

NADH carries XX → ETC → ATP production

NAD⁺ and NADH: Electron ShuttlesWhat is NAD⁺?

NAD⁺ = Nicotinamide Adenine Dinucleotide

NAD⁺ is an electron shuttle used during cellular respiration.

Its main job is to:

• pick up electrons and hydrogen removed from food

• carry them to the electron transport chain

• return and pick up more electrons again

We make NAD⁺ using vitamin B3 (niacin) from our diet.

Why is it called a dinucleotide?

NAD⁺ is made of two nucleotides, which is why it is called a dinucleotide.

One important part contains the nitrogenous base:

nicotinamide

The nicotinamide part is where electrons and hydrogen are accepted.

How NAD⁺ Works During Cellular Respiration

As food is broken down, hydrogen atoms are removed from it.

They are generally removed:

2 hydrogen atoms at a time

The enzymes that remove these hydrogens are called:

dehydrogenase enzymes

Dehydrogenase enzymes remove hydrogen from food molecules.

What happens to the 2 hydrogen atoms?

Two hydrogen atoms contain:

2 H⁺ + 2 e⁻

NAD⁺ accepts:

• 2 electrons

• 1 proton (H⁺)

This changes NAD⁺ into:

NADH

The other proton is released as:

H⁺

So the simplified reaction is:

NAD⁺ + 2H → NADH + H⁺

Another way to think about it:

NAD⁺ picks up:

• one whole hydrogen atom = H⁺ + e⁻

• one additional electron from the second hydrogen

So NADH carries:

2 electrons + 1 H⁺

The other H⁺ remains separate.

NAD⁺ vs. NADHNAD⁺

• oxidized form

• has not picked up the electrons yet

• acts like an empty electron shuttle

• positively charged

NADH

• reduced form

• has gained electrons

• carries high-energy electrons

• acts like a loaded electron shuttle

Remember:

GER = Gain Electrons → Reduced

So:

NAD⁺ gains electrons → NADH

That means NAD⁺ is reduced when it becomes NADH.

What Happens to NADH?

NADH carries its electrons to the:

mitochondrial electron transport chain

At the ETC:

• NADH gives up its electrons

• the energy from those electrons helps create the H⁺ gradient

• that gradient is used to make ATP

After NADH gives away its electrons, it becomes:

NAD⁺ again

Then NAD⁺ can go back and pick up more electrons.

So the cycle is:

NAD⁺ → picks up electrons → NADH → delivers electrons to ETC → NAD⁺

Oxidation and Reduction

When:

NAD⁺ → NADH

NAD⁺ gains electrons, so it is:

reduced

When:

NADH → NAD⁺

NADH loses electrons, so it is:

oxidized

This follows:

LEO-GER

• Lose Electrons = Oxidized

• Gain Electrons = Reduced

Structural Change Your Professor MentionedNAD⁺

• oxidized form

• nicotinamide ring is positively charged

• has 3 double bonds in the ring

NADH

• reduced form

• accepts 2 electrons and 1 proton

• is no longer positively charged

• has 2 double bonds in the ring

The main thing to remember is:

NAD⁺ = oxidized form

NADH = reduced form

Other Electron Shuttles

Two similar electron carriers are:

• FAD → used in cellular respiration

• NADPH → used mainly in photosynthesis

Important Summary

NAD⁺

• oxidized form

• empty shuttle

• accepts electrons and hydrogen

• becomes NADH

NADH

• reduced form

• loaded shuttle

• carries electrons to the ETC

• helps with ATP production

Dehydrogenase enzymes

• remove hydrogen from food

Best shortcut

NAD⁺ picks up electrons → NADH

NADH carries electrons → ETC → ATP production

Glycolysis: Part I — Prep / Energy Investment PhaseWhat is glycolysis?

Glycolysis = XX XX sugar

More specifically, XX is gradually broken down into smaller molecules.

Where does glycolysis happen?

Glycolysis happens in the:

XX

This occurs in:

• bacteria

• fungi

• plants

• protists

• animals

So glycolysis does not require a XX

Glycolysis has 10 steps and 10 enzymes

Your professor emphasized:

10 steps, 10 enzymes

She also said you especially need to know the enzymes that are kinases.

A kinase is an XX that transfers a XX XX, usually from ATP, onto another molecule.

In the prep phase, the two important kinase enzymes are:

Step 1: XX

Step 3: XX

Step 1: Glucose → Glucose-X-XX

Enzyme:

XX

Reaction:

XX → Glucose-X XX

Hexokinase is a kinase, so its job is to transfer a XX XX

In this step:

• XX donates a phosphate group

• glucose receives the XX

• glucose becomes glucose-X-phosphate

• XX becomes XX

So:

Glucose + ATP → Glucose-6-phosphate + ADP

This step is endergonic by itself, meaning it requires XX

The cell couples it with ATP breakdown, which provides enough energy to make the overall reaction favorable.

The phosphate makes glucose:

more XX XX

This creates a:

XX XX

So the main thing to remember is:

Hexokinase adds a XX from XX to XX

Why is it called hexokinase?

“Hexo” refers to a 6-carbon sugar, and “kinase” means it transfers a phosphate.

So hexokinase phosphorylates XX, which is a 6-carbon sugar.

Step 2: Glucose-6-phosphate → XX-6-phosphate

Enzyme:

Phosphoglucose isomerase

Reaction:

Glucose-6-phosphate → Fructose-6-phosphate

This step:

• does not require XX

• rearranges the XX

• is an XXX reaction

Your professor described this as an XX step.

No ATP is invested here.

Step 3: Fructose-6-phosphate → Fructose-X ,X-XX

Enzyme:

XX

This is the second major kinase in the prep phase.

Phosphofructokinase transfers another XX XX from ATP onto fructose-6-phosphate.

So:

Fructose-6-phosphate + ATP → Fructose-1,6-bisphosphate + ADP

In this step:

• a second XX is used

• another XX is added

• the molecule becomes even more XX and XX

The product is:

Fructose-1,6-bisphosphate

So the key thing to remember is:

Phosphofructokinase adds a XX phosphate fromXX to XX-6-phosphate.

This prepares the molecule to be split into two smaller XX

Why is fructose-1,6-bisphosphate important?

It has:

• one phosphate on carbon 1

• one phosphate on carbon 6

Your professor described these two phosphate groups like:

“handlebars on a bike”

The two phosphate groups make the molecule highly XX and ready to be broken apart.

Step 4: Splitting the 6-carbon molecule

Enzyme:

Aldolase

Fructose-1,6-bisphosphate is split into XX 3-XX molecules:

• Glyceraldehyde-3-phosphate (XX)

• Dihydroxyacetone phosphate (XX)

So the original 6-carbon sugar is now split into:

XX X-carbon molecules

Step 5: DHAP → G3P

Enzyme:

Isomerase

The DHAP molecule is converted into:

Glyceraldehyde-3-phosphate (G3P)

So by the end of the prep phase, you have:

X G3P molecules

Both G3P molecules continue into the next part of glycolysis.

Energy Investment Phase

The first half of glycolysis is called the:

XX XX XX

because the cell has to spend XX

Total ATP used:

X X

• 1 ATP used by XX in Step X

• 1 ATP used by XX in Step X

The purpose of spending ATP is to phosphorylate XX and make it more reactive so it can eventually be split and produce energy.

Overall result of the prep phase

Start with:

1 glucose

Use:

2 ATP

End with:

2 G3P

Important Enzyme Idea

The arrow in each step represents a chemical reaction.

The enzyme written under the arrow is the enzyme that catalyzes that reaction.

So:

substrate → product

and the enzyme under the arrow tells you what protein performs that chemical change.

Kinases You Need to KnowHexokinase — Step X

• kinase enzyme

• transfers phosphate from XX to XX

• uses X ATP

• XX → XX X XX

• makes glucose more X

Phosphofructokinase — Step X

• kinase enzyme

• transfers phosphate from XX to fructose-X XX

• uses X ATP

• fructose-6-phosphate → fructose-X,X-XX

• makes the molecule more unstable/reactive so it can be XX

Important definition

Kinase = enzyme that transfers a XX group, usually from ATP, to another molecule.

Important Things to Memorize

Glycolysis

• breaks down XX

• happens in the XX

• has 10 steps and 10 enzymes

Step 1

• enzyme = hexokinase

• kinase

• uses 1 ATP

• adds phosphate to glucose

• glucose → glucose-6-phosphate

Step 2

• glucose-6-phosphate → fructose-6-phosphate

• no ATP used

Step 3

• enzyme = phosphofructokinase

• kinase

• uses 1 ATP

• adds another phosphate

• fructose-6-phosphate → fructose-1,6-bisphosphate

Step 4

• fructose-1,6-bisphosphate is split

• produces G3P + DHAP

Step 5

• DHAP is converted to G3P

End result:

X glucose + X ATP → X G3P

Glycolysis: Part I — Prep / Energy Investment PhaseWhat is glycolysis?

Glycolysis = breaking down sugar

More specifically, glucose is gradually broken down into smaller molecules.

Where does glycolysis happen?

Glycolysis happens in the:

cytoplasm

This occurs in:

• bacteria

• fungi

• plants

• protists

• animals

So glycolysis does not require a mitochondrion.

Glycolysis has 10 steps and 10 enzymes

Your professor emphasized:

10 steps, 10 enzymes

She also said you especially need to know the enzymes that are kinases.

A kinase is an enzyme that transfers a phosphate group, usually from ATP, onto another molecule.

In the prep phase, the two important kinase enzymes are:

Step 1: Hexokinase

Step 3: Phosphofructokinase

Step 1: Glucose → Glucose-6-phosphate

Enzyme:

Hexokinase

Reaction:

Glucose → Glucose-6-phosphate

Hexokinase is a kinase, so its job is to transfer a phosphate group.

In this step:

• ATP donates a phosphate group

• glucose receives the phosphate

• glucose becomes glucose-6-phosphate

• ATP becomes ADP

So:

Glucose + ATP → Glucose-6-phosphate + ADP

This step is endergonic by itself, meaning it requires energy.

The cell couples it with ATP breakdown, which provides enough energy to make the overall reaction favorable.

The phosphate makes glucose:

more chemically reactive

This creates a:

phosphorylated intermediate

So the main thing to remember is:

Hexokinase adds a phosphate from ATP to glucose.

Why is it called hexokinase?

“Hexo” refers to a 6-carbon sugar, and “kinase” means it transfers a phosphate.

So hexokinase phosphorylates glucose, which is a 6-carbon sugar.

Step 2: Glucose-6-phosphate → Fructose-6-phosphate

Enzyme:

Phosphoglucose isomerase

Reaction:

Glucose-6-phosphate → Fructose-6-phosphate

This step:

• does not require ATP

• rearranges the molecule

• is an isomerization reaction

Your professor described this as an exergonic step.

No ATP is invested here.

Step 3: Fructose-6-phosphate → Fructose-1,6-bisphosphate

Enzyme:

Phosphofructokinase

This is the second major kinase in the prep phase.

Phosphofructokinase transfers another phosphate group from ATP onto fructose-6-phosphate.

So:

Fructose-6-phosphate + ATP → Fructose-1,6-bisphosphate + ADP

In this step:

• a second ATP is used

• another phosphate is added

• the molecule becomes even more reactive and unstable

The product is:

Fructose-1,6-bisphosphate

So the key thing to remember is:

Phosphofructokinase adds a second phosphate from ATP to fructose-6-phosphate.

This prepares the molecule to be split into two smaller molecules.

Why is fructose-1,6-bisphosphate important?

It has:

• one phosphate on carbon 1

• one phosphate on carbon 6

Your professor described these two phosphate groups like:

“handlebars on a bike”

The two phosphate groups make the molecule highly reactive and ready to be broken apart.

Step 4: Splitting the 6-carbon molecule

Enzyme:

Aldolase

Fructose-1,6-bisphosphate is split into two 3-carbon molecules:

• Glyceraldehyde-3-phosphate (G3P)

• Dihydroxyacetone phosphate (DHAP)

So the original 6-carbon sugar is now split into:

two 3-carbon molecules

Step 5: DHAP → G3P

Enzyme:

Isomerase

The DHAP molecule is converted into:

Glyceraldehyde-3-phosphate (G3P)

So by the end of the prep phase, you have:

2 G3P molecules

Both G3P molecules continue into the next part of glycolysis.

Energy Investment Phase

The first half of glycolysis is called the:

Energy Investment Phase

because the cell has to spend ATP.

Total ATP used:

2 ATP

• 1 ATP used by hexokinase in Step 1

• 1 ATP used by phosphofructokinase in Step 3

The purpose of spending ATP is to phosphorylate glucose and make it more reactive so it can eventually be split and produce energy.

Overall result of the prep phase

Start with:

1 glucose

Use:

2 ATP

End with:

2 G3P

Important Enzyme Idea

The arrow in each step represents a chemical reaction.

The enzyme written under the arrow is the enzyme that catalyzes that reaction.

So:

substrate → product

and the enzyme under the arrow tells you what protein performs that chemical change.

Kinases You Need to KnowHexokinase — Step 1

• kinase enzyme

• transfers phosphate from ATP to glucose

• uses 1 ATP

• glucose → glucose-6-phosphate

• makes glucose more reactive

Phosphofructokinase — Step 3

• kinase enzyme

• transfers phosphate from ATP to fructose-6-phosphate

• uses 1 ATP

• fructose-6-phosphate → fructose-1,6-bisphosphate

• makes the molecule more unstable/reactive so it can be split

Important definition

Kinase = enzyme that transfers a phosphate group, usually from ATP, to another molecule.

Important Things to Memorize

Glycolysis

• breaks down glucose

• happens in the cytoplasm

• has 10 steps and 10 enzymes

Step 1

• enzyme = hexokinase

• kinase

• uses 1 ATP

• adds phosphate to glucose

• glucose → glucose-6-phosphate

Step 2

• glucose-6-phosphate → fructose-6-phosphate

• no ATP used

Step 3

• enzyme = phosphofructokinase

• kinase

• uses 1 ATP

• adds another phosphate

• fructose-6-phosphate → fructose-1,6-bisphosphate

Step 4

• fructose-1,6-bisphosphate is split

• produces G3P + DHAP

Step 5

• DHAP is converted to G3P

End result:

1 glucose + 2 ATP → 2 G3P

Figure 9.9a: Glycolysis: Part II – the Energy

Payoff Phase, thanks to the last 2 Kinases

Part 2: Energy Capture Phase:

a) Oxidation: 2 NADH + 2 H+

b) Substrate-level phosphorylation:

4 ATPs produced = 2 NET ATPs)

c) [Dehydration: 2 molecules of H2O]

Glycolysis: Part II — Energy Payoff PhaseBig picture

This is the second half of glycolysis.

The first half used XX XX to prepare glucose.
Now the cell starts getting energy back.

The payoff phase:

• removes XX and XX

• makes XX

• makes XX

• ends with XX

Remember that after Part I, one glucose has become:

2 glyceraldehyde-3-phosphate (XX)

Because there are 2 G3P molecules, every reaction in Part II happens XX per glucose.

Step 6: G3P is XX

Enzyme:

XX XX dehydrogenase

This is one of the important dehydrogenase enzymes.

A dehydrogenase removes XX /electrons from a molecule.

In this step:

• XX /electrons are removed from XX

• XX accepts those electrons

• NAD⁺ becomes XX

• this happens XX because there are 2 XX molecules

So the cell produces:

2 NADH + 2 H⁺

This is the first point in glycolysis where hydrogen from the original glucose is being removed.

Your professor emphasized that:

X of the original 12 hydrogens from glucose are removed during XX

The product formed is:

XX

Important redox idea

G3P loses electrons/hydrogen, so G3P is:

XX

NAD⁺ gains electrons, so NAD⁺ is:

XX to NADH

Step 7: First ATP production

Enzyme:

XX XX

This is another kinase you need to know.

But this time, instead of ATP donating a phosphate, the phosphate is transferred from the substrate to XX.

Because there are 2 molecules of 1,3-bisphosphoglycerate:

• 2 phosphate groups are XX

• 2 XX become XX XX

The product is:

XX XX

This process is called:

xx-xx phosphorylation

This means:

A phosphate is transferred directly from a phosphorylated substrate to xx to make xx.

So in Step 7:

1,3-bisphosphoglycerate + ADP → 3-phosphoglycerate + ATP

Since this happens twice:

XX XX are produced

At this point, the cell has replaced the 2 ATP it spent in the XX XX

So the net ATP is:

X so far

Step 8: Rearrangement

Enzyme:

Phosphoglyceromutase

The phosphate group is moved to a different XX.

Reaction:

3-phosphoglycerate → 2-phosphoglycerate

No XX is used or produced here.

Step 9: Dehydration

Enzyme:

Enolase

This step removes XX

Reaction:

2-phosphoglycerate → phosphoenolpyruvate (PEP)

Because there are two molecules:

XX XX are released

This is called a:

XX XX

The product is:

phosphoenolpyruvate (XX)

PEP is a very high-energy phosphorylated molecule.

Step 10: Second ATP production

Enzyme:

XX XX

This is another important kinase.

Pyruvate kinase removes the remaining XX from PEP and transfers it to XX.

Reaction:

PEP + ADP → pyruvate + ATP

Because there are 2 PEP molecules:

X ATP are produced

The final product of glycolysis is:

X pyruvate

This is the first point where glycolysis gives you a net ATP gain.

ATP accountingPrep phase

Used:

2 ATP

Payoff phase

Produced:

4 ATP total

• Step 7 produces 2 ATP

• Step 10 produces 2 ATP

So:

X ATP produced − X ATP used = X net ATP

Therefore glycolysis produces:

X net ATP per XX

Kinases you need to know in Part II XX XX

— Step 7

• transfers XX from the substrate to XX

• makes XX

• X ATP are produced because there are 2 substrate molecules

• this is XX XX phosphorylation

XX XX— Step 10

• transfers XX from XX to XX

• produces XX

• produces X ATP total

• converts XX into XX

• this is also XX-XX phosphorylation

So the four important glycolysis kinases are:

• XX— Step 1

• XX— Step 3

• XX XX — Step 7

• XX — Step 10

The first two XX ATP.

The last two XX ATP.

That distinction is very important.

End Products of Glycolysis

From one glucose, glycolysis produces:

• X pyruvate

• X NADH

• X H⁺

• X ATP produced

• X ATP used

• X net ATP

• X H₂O

Hydrogen tracking

Glucose started with:

XX hydrogens

During glycolysis:

X hydrogens are removed

Those electrons/hydrogens are captured by NAD⁺ to form:

2 NADH

So after glycolysis:

2 of the 12 hydrogens have been removed

Best shortcut

Part I = spend 2 ATP

Part II = make 4 ATP + 2 NADH

Therefore:

Net glycolysis = 2 ATP + 2 NADH + 2 pyruvate

And for the kinase enzymes:

Steps 1 & 3 → use ATP

Steps 7 & 10 → make ATP

Part 2: Energy Capture Phase:

a) Oxidation: 2 NADH + 2 H+

b) Substrate-level phosphorylation:

4 ATPs produced = 2 NET ATPs)

c) [Dehydration: 2 molecules of H2O]

Glycolysis: Part II — Energy Payoff PhaseBig picture

This is the second half of glycolysis.

The first half used 2 ATP to prepare glucose.
Now the cell starts getting energy back.

The payoff phase:

• removes electrons and hydrogen

• makes NADH

• makes ATP

• ends with pyruvate

Remember that after Part I, one glucose has become:

2 glyceraldehyde-3-phosphate (G3P)

Because there are 2 G3P molecules, every reaction in Part II happens twice per glucose.

Step 6: G3P is oxidized

Enzyme:

Triose phosphate dehydrogenase

This is one of the important dehydrogenase enzymes.

A dehydrogenase removes hydrogen/electrons from a molecule.

In this step:

• hydrogens/electrons are removed from G3P

• NAD⁺ accepts those electrons

• NAD⁺ becomes NADH

• this happens twice because there are 2 G3P molecules

So the cell produces:

2 NADH + 2 H⁺

This is the first point in glycolysis where hydrogen from the original glucose is being removed.

Your professor emphasized that:

2 of the original 12 hydrogens from glucose are removed during glycolysis

The product formed is:

1,3-bisphosphoglycerate

Important redox idea

G3P loses electrons/hydrogen, so G3P is:

oxidized

NAD⁺ gains electrons, so NAD⁺ is:

reduced to NADH

Step 7: First ATP production

Enzyme:

Phosphoglycerate kinase

This is another kinase you need to know.

But this time, instead of ATP donating a phosphate, the phosphate is transferred from the substrate to ADP.

Because there are 2 molecules of 1,3-bisphosphoglycerate:

• 2 phosphate groups are removed

• 2 ADP become 2 ATP

The product is:

3-phosphoglycerate

This process is called:

substrate-level phosphorylation

Substrate-level phosphorylation

This means:

A phosphate is transferred directly from a phosphorylated substrate to ADP to make ATP.

So in Step 7:

1,3-bisphosphoglycerate + ADP → 3-phosphoglycerate + ATP

Since this happens twice:

2 ATP are produced

At this point, the cell has replaced the 2 ATP it spent in the prep phase.

So the net ATP is:

0 so far

Step 8: Rearrangement

Enzyme:

Phosphoglyceromutase

The phosphate group is moved to a different carbon.

Reaction:

3-phosphoglycerate → 2-phosphoglycerate

No ATP is used or produced here.

Step 9: Dehydration

Enzyme:

Enolase

This step removes water.

Reaction:

2-phosphoglycerate → phosphoenolpyruvate (PEP)

Because there are two molecules:

2 H₂O are released

This is called a:

dehydration reaction

The product is:

phosphoenolpyruvate (PEP)

PEP is a very high-energy phosphorylated molecule.

Step 10: Second ATP production

Enzyme:

Pyruvate kinase

This is another important kinase.

Pyruvate kinase removes the remaining phosphate from PEP and transfers it to ADP.

Reaction:

PEP + ADP → pyruvate + ATP

Because there are 2 PEP molecules:

2 ATP are produced

The final product of glycolysis is:

2 pyruvate

This is the first point where glycolysis gives you a net ATP gain.

ATP accountingPrep phase

Used:

2 ATP

Payoff phase

Produced:

4 ATP total

• Step 7 produces 2 ATP

• Step 10 produces 2 ATP

So:

4 ATP produced − 2 ATP used = 2 net ATP

Therefore glycolysis produces:

2 net ATP per glucose

Kinases you need to know in Part IIPhosphoglycerate kinase — Step 7

• transfers phosphate from the substrate to ADP

• makes ATP

• 2 ATP are produced because there are 2 substrate molecules

• this is substrate-level phosphorylation

Pyruvate kinase — Step 10

• transfers phosphate from PEP to ADP

• produces ATP

• produces 2 ATP total

• converts PEP into pyruvate

• this is also substrate-level phosphorylation

So the four important glycolysis kinases are:

• Hexokinase — Step 1

• Phosphofructokinase — Step 3

• Phosphoglycerate kinase — Step 7

• Pyruvate kinase — Step 10

The first two use ATP.

The last two make ATP.

That distinction is very important.

End Products of Glycolysis

From one glucose, glycolysis produces:

• 2 pyruvate

• 2 NADH

• 2 H⁺

• 4 ATP produced

• 2 ATP used

• 2 net ATP

• 2 H₂O

Hydrogen tracking

Glucose started with:

12 hydrogens

During glycolysis:

2 hydrogens are removed

Those electrons/hydrogens are captured by NAD⁺ to form:

2 NADH

So after glycolysis:

2 of the 12 hydrogens have been removed

Best shortcut

Part I = spend 2 ATP

Part II = make 4 ATP + 2 NADH

Therefore:

Net glycolysis = 2 ATP + 2 NADH + 2 pyruvate

And for the kinase enzymes:

Steps 1 & 3 → use ATP

Steps 7 & 10 → make ATP

Kinases in Glycolysis

Your professor is emphasizing that you need to know all 4 kinase enzymes in glycolysis.

A kinase transfers a XX XX

In glycolysis, the kinases do two different jobs:

• during the energy investment phase, they use XX to add XX XX

• during the energy payoff phase, they help transfer XX XX to XX to make XX

Energy Investment Phase1. Hexokinase (HK)

Hexokinase acts on:

XX

It transfers a XX from XX to XX

So:

Glucose + ATP → Glucose-6-phosphate + ADP

What happens:

• glucose receives its first XX

• X ATP is used

• glucose becomes glucose-X-phosphate

• XX makes glucose more chemically reactive

Main idea:

Hexokinase adds the first phosphate.

2. Phosphofructokinase (PFK)

Phosphofructokinase acts on:

XX-X-phosphate

This molecule already has one phosphate, and PFK adds a second phosphate using anotherXX

So:

Fructose-6-phosphate + ATP → Fructose-1,6-bisphosphate + ADP

What happens:

• a second XX is used

• a second XX is added

• fructose-1,6-bisphosphate is formed

• the molecule becomes very XX and ready to XX

Your professor calls the two phosphate groups:

“Handlebars”

Main idea:

Phosphofructokinase adds the second phosphate.

Energy investment total

You put in:

XX XX

• 1 ATP used by hexokinase

• 1 ATP used by phosphofructokinase

Energy Payoff Phase

At this point, glucose has already been split into:

XX X-carbon molecules

Because there are now two molecules, each ATP-producing reaction occurs XX

3. Phosphoglycerate Kinase (PGK)

Phosphoglycerate kinase helps produce XX by:

XX XX XX

A phosphate is transferred directly from the XX to XX.

Because there are two 3-carbon molecules:

X ATP are produced

Main idea:

Phosphoglycerate kinase makes X ATP.

4. Pyruvate Kinase (PK)

Pyruvate kinase acts near the end of glycolysis.

It transfers the phosphate from XX to XX

This produces:

• 2 ATP

• 2 pyruvate

Because there are two PEP molecules:

X ATP are produced

Main idea:

Pyruvate kinase makes another X XX

XX XX XX

This is important for both payoff-phase kinases.

Substrate-level phosphorylation means:

A phosphate is transferred directly from a XX molecule to XX, producing XX

So:

phosphorylated substrate + ADP → product + ATP

This happens at:

• phosphoglycerate kinase

• pyruvate kinase

ATP AccountingEnergy investment

2 ATP used

Energy payoff

4 ATP produced

• 2 from phosphoglycerate kinase

• 2 from pyruvate kinase

So:

4 ATP produced − 2 ATP used = X net ATP

The 4 Kinases You Need to KnowEnergy Investment

Hexokinase

• adds first XX

• uses X ATP

Phosphofructokinase

• adds second XX

• uses X ATP

Energy Payoff

Phosphoglycerate kinase

• makes X ATP

• substrate-level phosphorylation

Pyruvate kinase

• makes X ATP

• substrate-level phosphorylation

Best shortcut

HK + PFK = ATP goes IN

PGK + PK = ATP comes OUT

So overall:

2 ATP in → 4 ATP out → 2 net ATP

Kinases in Glycolysis

Your professor is emphasizing that you need to know all 4 kinase enzymes in glycolysis.

A kinase transfers a phosphate group.

In glycolysis, the kinases do two different jobs:

• during the energy investment phase, they use ATP to add phosphate groups

• during the energy payoff phase, they help transfer phosphate groups to ADP to make ATP

Energy Investment Phase1. Hexokinase (HK)

Hexokinase acts on:

Glucose

It transfers a phosphate from ATP to glucose.

So:

Glucose + ATP → Glucose-6-phosphate + ADP

What happens:

• glucose receives its first phosphate

• 1 ATP is used

• glucose becomes glucose-6-phosphate

• phosphorylation makes glucose more chemically reactive

Main idea:

Hexokinase adds the first phosphate.

2. Phosphofructokinase (PFK)

Phosphofructokinase acts on:

Fructose-6-phosphate

This molecule already has one phosphate, and PFK adds a second phosphate using another ATP.

So:

Fructose-6-phosphate + ATP → Fructose-1,6-bisphosphate + ADP

What happens:

• a second ATP is used

• a second phosphate is added

• fructose-1,6-bisphosphate is formed

• the molecule becomes very reactive and ready to split

Your professor calls the two phosphate groups:

“Handlebars”

Main idea:

Phosphofructokinase adds the second phosphate.

Energy investment total

You put in:

2 ATP

• 1 ATP used by hexokinase

• 1 ATP used by phosphofructokinase

Energy Payoff Phase

At this point, glucose has already been split into:

two 3-carbon molecules

Because there are now two molecules, each ATP-producing reaction occurs twice.

3. Phosphoglycerate Kinase (PGK)

Phosphoglycerate kinase helps produce ATP by:

substrate-level phosphorylation

A phosphate is transferred directly from the substrate to ADP.

Because there are two 3-carbon molecules:

2 ATP are produced

Main idea:

Phosphoglycerate kinase makes 2 ATP.

4. Pyruvate Kinase (PK)

Pyruvate kinase acts near the end of glycolysis.

It transfers the phosphate from PEP to ADP.

This produces:

• ATP

• pyruvate

Because there are two PEP molecules:

2 ATP are produced

Main idea:

Pyruvate kinase makes another 2 ATP.

Substrate-Level Phosphorylation

This is important for both payoff-phase kinases.

Substrate-level phosphorylation means:

A phosphate is transferred directly from a phosphorylated molecule to ADP, producing ATP.

So:

phosphorylated substrate + ADP → product + ATP

This happens at:

• phosphoglycerate kinase

• pyruvate kinase

ATP AccountingEnergy investment

2 ATP used

Energy payoff

4 ATP produced

• 2 from phosphoglycerate kinase

• 2 from pyruvate kinase

So:

4 ATP produced − 2 ATP used = 2 net ATP

The 4 Kinases You Need to KnowEnergy Investment

Hexokinase

• adds first phosphate

• uses 1 ATP

Phosphofructokinase

• adds second phosphate

• uses 1 ATP

Energy Payoff

Phosphoglycerate kinase

• makes 2 ATP

• substrate-level phosphorylation

Pyruvate kinase

• makes 2 ATP

• substrate-level phosphorylation

Best shortcut

HK + PFK = ATP goes IN

PGK + PK = ATP comes OUT

So overall:

2 ATP in → 4 ATP out → 2 net ATP

Figure 9.10 Formation of Acetyl CoA, the junction

between glycolysis and the citric acid cycle

Where: from cytoplasm into mitochondria

Overall: 2 Pyruvate (3C) --> 2 Acetyl CoA (2C) + 2 CO2 + 2 NADH

The summary name for the following is: Oxidative decarboxylation:

(1) Pyruvate is de-carboxylated (loss of CO2),

(2) Oxidized (H transferred to NAD+  reduced to NADH);

(3) The remaining 2 C molecule is attached to CoA (a B-vitamin-like molecule with ~S-bond)

Formation of Acetyl-CoAWhat is this step?

The formation of Acetyl-CoA is the step that connects:

XX → XX XX / Citric Acid Cycle

It happens after XX and before the XX XX.

Where does it happen?

Glycolysis happens in the:

XX

Pyruvate then moves into the:

XX

The actual Acetyl-CoA formation reaction occurs in the:

XX XX

The matrix is the liquid-filled part inside the mitochondrion.

Also remember:

XX XX happens in the matrix

When can this happen?

This pathway continues when the organism is carrying out XX XX.

In eukaryotic cells:

• XX enters the XX

• this step continues as long as XX XX can continue

Aerobic bacteria do not have XX, but they can still perform similar aerobic reactions using bacterial cell structures.

Overall Reaction

From one glucose, glycolysis made:

X pyruvate

Each pyruvate has:

X carbons

Overall:

2 Pyruvate (3C) → 2 Acetyl-CoA (2C) + 2 CO₂ + 2 NADH

So this step does three important things:

• removes XX

• removes XX/electrons

• attaches the remaining 2-carbon molecule to XX XX

Name of This Process

XX XX

That name describes the two major things happening.

Decarboxylation

Decarboxylation = removal of XX

Each pyruvate loses:

1 XX as CO₂

Since there are 2 pyruvate molecules:

X CO₂ are released total

Oxidation

The pyruvate molecule also loses:

XX XX

So pyruvate is:

XX

NAD⁺ accepts those electrons/hydrogen and becomes:

XX

So:

NAD⁺ → XX

NAD⁺ is reduced because it gains XX

Enzyme You Need to Know

The enzyme complex responsible for this reaction is:

XX XX XX (PDC).

It helps convert:

XX → XX

Why is it called a dehydrogenase?

A dehydrogenase removes XX/ electrons from a molecule.

During this step:

• pyruvate loses XX/electrons

• XX picks them up

• XX is produced

Step 1: Pyruvate is XX

Each pyruvate starts with:

X carbons

One carbon is removed as:

XX

So:

3-carbon pyruvate → 2-carbon molecule

Since this happens twice:

X CO₂ total are released

These are the first:

X of the original X carbons from glucose

to leave the pathway as XX

Step 2: Pyruvate is XX

The remaining molecule loses:

• XX

• XX

XX accepts them.

So:

NAD⁺ → NADH

Because there are 2 pyruvate molecules:

2 XX are produced

Your professor described this as removing:

2 more XX

from the original glucose molecule.

Step 3: XX X is Added

The remaining:

XX XX molecule

is attached to:

Coenzyme A (CoA)

This forms:

XX XX

Coenzyme A is described on the slide as a:

B-vitamin-like molecule

It also contains a sulfur-containing bond/group.

Acetyl-CoA is now ready to enter the:

XX XX

Carbon Tracking

Glucose originally had:

X carbons

After glycolysis:

2 pyruvate × 3 carbons = X carbons

During Acetyl-CoA formation:

X carbons are removed as 2 CO₂

That leaves:

X carbons total

in the:

2 Acetyl-CoA molecules

Those remaining carbons will enter the XX XX

Hydrogen Tracking

Glucose started with:

XX hydrogens

During glycolysis:

X hydrogens were removed

During Acetyl-CoA formation:

X more hydrogens are removed

So by the end of this step:

X of the original 12 hydrogens have been removed

That means:

X hydrogens are still left to remove

Those remaining hydrogens will mainly be removed during the XX XX.

Products Per Glucose

Because one glucose makes 2 pyruvate, this reaction happens twice.

Final products are:

• X Acetyl-CoA

• X CO₂

• X NADH

There is:

no XX made directly in this step

Why Oxygen Matters

Your professor emphasized that this process is tied to aerobic respiration.

If aerobic respiration is continuing:

• pyruvate can move into the mitochondrion

• Acetyl-CoA can form

• the Krebs cycle can continue

• NADH can later deliver electrons to the electron transport chain

Oxygen is not directly used in the Acetyl-CoA reaction itself, but the pathway depends on oxygen being available so the later electron transport steps can continue.

Pyruvate Dehydrogenase Complex Deficiency (PDCD)

This is a:

XX XX

The slide also calls it:

congenital infantile lactic acidosis

If the pyruvate dehydrogenase complex does not work properly:

• pyruvate cannot efficiently become XX XX

• pyruvate may instead be converted to XX

• XX can build up

• this can cause XX XX

Most Important Things to Know

Location

• pyruvate comes from XX

• Acetyl-CoA formation happens in XX XX

• XX XX also happens in matrix

Enzyme

• XX XX

What happens

• pyruvate loses XX

• pyruvate is XX

• XX becomes XX

• XX is attached

• XX XX formed

Overall
2 Pyruvate → 2 Acetyl-CoA + 2 CO₂ + 2 NADH

Carbon tracking

• 2 of 6 carbons are gone as CO₂

• X carbons remain

Hydrogen tracking

• 2 H lost in glycolysis

• 2 more H lost here

• X H still remain

Disease to know

• PDCD

• congenital infantile lactic acidosis

Best shortcut

Pyruvate → lose CO₂ → lose H/e⁻ → add CoA → Acetyl-CoA

Decarboxylation = lose CO₂

Oxidation = lose electrons/hydrogen

PDC = Pyruvate Dehydrogenase Complex

Figure 9.10 Formation of Acetyl CoA, the junction

between glycolysis and the citric acid cycle

Where: from cytoplasm into mitochondria

Overall: 2 Pyruvate (3C) --> 2 Acetyl CoA (2C) + 2 CO2 + 2 NADH

The summary name for the following is: Oxidative decarboxylation:

(1) Pyruvate is de-carboxylated (loss of CO2),

(2) Oxidized (H transferred to NAD+  reduced to NADH);

(3) The remaining 2 C molecule is attached to CoA (a B-vitamin-like molecule with ~S-bond)

Formation of Acetyl-CoAWhat is this step?

The formation of Acetyl-CoA is the step that connects:

Glycolysis → Krebs cycle / Citric Acid Cycle

It happens after glycolysis and before the Krebs cycle.

Where does it happen?

Glycolysis happens in the:

cytoplasm

Pyruvate then moves into the:

mitochondrion

The actual Acetyl-CoA formation reaction occurs in the:

mitochondrial matrix

The matrix is the liquid-filled part inside the mitochondrion.

Also remember:

Krebs cycle happens in the matrix

When can this happen?

This pathway continues when the organism is carrying out aerobic respiration.

In eukaryotic cells:

• pyruvate enters the mitochondrion

• this step continues as long as aerobic respiration can continue

Aerobic bacteria do not have mitochondria, but they can still perform similar aerobic reactions using bacterial cell structures.

Overall Reaction

From one glucose, glycolysis made:

2 pyruvate

Each pyruvate has:

3 carbons

Overall:

2 Pyruvate (3C) → 2 Acetyl-CoA (2C) + 2 CO₂ + 2 NADH

So this step does three important things:

• removes CO₂

• removes hydrogen/electrons

• attaches the remaining 2-carbon molecule to Coenzyme A

Name of This Process

The slide calls this process:

Oxidative decarboxylation

That name describes the two major things happening.

Decarboxylation

Decarboxylation = removal of CO₂

Each pyruvate loses:

1 carbon as CO₂

Since there are 2 pyruvate molecules:

2 CO₂ are released total

Oxidation

The pyruvate molecule also loses:

• hydrogen

• electrons

So pyruvate is:

oxidized

NAD⁺ accepts those electrons/hydrogen and becomes:

NADH

So:

NAD⁺ → NADH

NAD⁺ is reduced because it gains electrons.

Enzyme You Need to Know

The enzyme complex responsible for this reaction is:

Pyruvate Dehydrogenase Complex (PDC)

Your professor specifically said to remember this enzyme.

It helps convert:

Pyruvate → Acetyl-CoA

Why is it called a dehydrogenase?

A dehydrogenase removes hydrogen/electrons from a molecule.

During this step:

• pyruvate loses hydrogen/electrons

• NAD⁺ picks them up

• NADH is produced

Step 1: Pyruvate is Decarboxylated

Each pyruvate starts with:

3 carbons

One carbon is removed as:

CO₂

So:

3-carbon pyruvate → 2-carbon molecule

Since this happens twice:

2 CO₂ total are released

These are the first:

2 of the original 6 carbons from glucose

to leave the pathway as CO₂.

Step 2: Pyruvate is Oxidized

The remaining molecule loses:

• hydrogen

• electrons

NAD⁺ accepts them.

So:

NAD⁺ → NADH

Because there are 2 pyruvate molecules:

2 NADH are produced

Your professor described this as removing:

2 more hydrogens

from the original glucose molecule.

Step 3: Coenzyme A is Added

The remaining:

2-carbon molecule

is attached to:

Coenzyme A (CoA)

This forms:

Acetyl-CoA

Coenzyme A is described on the slide as a:

B-vitamin-like molecule

It also contains a sulfur-containing bond/group.

Acetyl-CoA is now ready to enter the:

Krebs cycle

Carbon Tracking

Glucose originally had:

6 carbons

After glycolysis:

2 pyruvate × 3 carbons = 6 carbons

During Acetyl-CoA formation:

2 carbons are removed as 2 CO₂

That leaves:

4 carbons total

in the:

2 Acetyl-CoA molecules

Those remaining carbons will enter the Krebs cycle.

Hydrogen Tracking

Glucose started with:

12 hydrogens

During glycolysis:

2 hydrogens were removed

During Acetyl-CoA formation:

2 more hydrogens are removed

So by the end of this step:

4 of the original 12 hydrogens have been removed

That means:

8 hydrogens are still left to remove

Those remaining hydrogens will mainly be removed during the Krebs cycle.

Products Per Glucose

Because one glucose makes 2 pyruvate, this reaction happens twice.

Final products are:

• 2 Acetyl-CoA

• 2 CO₂

• 2 NADH

There is:

no ATP made directly in this step

Why Oxygen Matters

Your professor emphasized that this process is tied to aerobic respiration.

If aerobic respiration is continuing:

• pyruvate can move into the mitochondrion

• Acetyl-CoA can form

• the Krebs cycle can continue

• NADH can later deliver electrons to the electron transport chain

Oxygen is not directly used in the Acetyl-CoA reaction itself, but the pathway depends on oxygen being available so the later electron transport steps can continue.

Pyruvate Dehydrogenase Complex Deficiency

Your professor specifically said to know:

Pyruvate Dehydrogenase Complex Deficiency (PDCD)

This is a:

mitochondrial disorder

The slide also calls it:

congenital infantile lactic acidosis

If the pyruvate dehydrogenase complex does not work properly:

• pyruvate cannot efficiently become Acetyl-CoA

• pyruvate may instead be converted to lactate

• lactate can build up

• this can cause lactic acidosis

Most Important Things to Know

Location

• pyruvate comes from cytoplasm

• Acetyl-CoA formation happens in mitochondrial matrix

• Krebs cycle also happens in matrix

Enzyme

• Pyruvate Dehydrogenase Complex

What happens

• pyruvate loses CO₂

• pyruvate is oxidized

• NAD⁺ becomes NADH

• CoA is attached

• Acetyl-CoA is formed

Overall
2 Pyruvate → 2 Acetyl-CoA + 2 CO₂ + 2 NADH

Carbon tracking

• 2 of 6 carbons are gone as CO₂

• 4 carbons remain

Hydrogen tracking

• 2 H lost in glycolysis

• 2 more H lost here

• 8 H still remain

Disease to know

• PDCD

• congenital infantile lactic acidosis

Best shortcut

Pyruvate → lose CO₂ → lose H/e⁻ → add CoA → Acetyl-CoA

Decarboxylation = lose CO₂

Oxidation = lose electrons/hydrogen

PDC = Pyruvate Dehydrogenase Complex

Krebs Cycle / Citric Acid Cycle

The Krebs cycle, also called the citric acid cycle or TCA cycle, happens in the XX XX, which is the liquid part of the mitochondrion. The Acetyl-CoA formation step happens before this, as pyruvate moves from the XX into the mitochondrion. The Krebs cycle itself does not directly use XX, but it is still part of aerobic respiration because XX and XXX need the electron transport chain to be recycled, and that ultimately depends on oxygen.

The reason it is called the citric acid cycle is because citric acid is one of the first products made when Acetyl-CoA enters the cycle. It is also called the TCA cycle, which stands for tricarboxylic acid cycle, because citric acid is a tricarboxylic acid.

From one glucose molecule, you get X Acetyl-CoA molecules, and each Acetyl-CoA contains 2 carbons, so a total of X carbons enter the Krebs cycle. The overall products from these 2 Acetyl-CoA molecules are 4 CO₂, 6 NADH, 2 FADH₂, and 2 ATP. The slide also shows 6 H⁺ being released along with the 6 NADH.

The main purpose of the Krebs cycle is not to make a large amount of ATP directly. Instead, the main purpose is to finish breaking down theXX XX and collect the remaining XX and XX. The cell wants to capture these electrons on the electron carriers XX and XX so they can later be used to make much more XX in the electron transport chain.

Glucose originally had X carbons. During Acetyl-CoA formation, X carbons were already removed as 2 CO₂, leaving X carbons to enter the Krebs cycle. During the Krebs cycle, these remaining 4 carbons are released as XX. By the end of the Krebs cycle, all 6 original carbons from glucose have been released as carbon dioxide, so the original glucose has essentially been completely broken down and the XX can be exhaled.

Glucose also originally had 12 hydrogens. During glycolysis, X hydrogens were removed, and during Acetyl-CoA formation, X more hydrogens were removed. That leaves X hydrogens to be collected during the Krebs cycle. By the end of the cycle, all X of these remaining hydrogens have been removed and their electrons have been captured by XX and XX.

During the Krebs cycle, XX picks up hydrogen and electrons and becomes XX. Per glucose, the Krebs cycle produces XX NADH. Since NADH has gained electrons, it is in the XX state. The cycle also uses another electron carrier called XX, which accepts hydrogen and electrons to become FADH₂. Per glucose, the Krebs cycle produces XX. These two carriers are important because they will carry the XX to the electron transport chain.

The Krebs cycle also makes X ATP total per glucose. These are only a small amount compared with the ATP that will be made later. The major ATP payoff comes during the XX XX chain and XX

When Acetyl-CoA enters the Krebs cycle, the XX portion comes back off. Its job is mainly to carry the X-carbon acetyl group into the cycle. The acetyl group continues through the cycle while CoA is released.

XX also happens in the Krebs cycle. Decarboxylation means removing XX. There are 2 decarboxylation reactions per turn of the cycle, and because the cycle runs twice per glucose, this gives a total of XX released.

The Krebs cycle has 8 steps and 8 enzymes. You do not need to know all 8 enzymes, but you do need to know X of them.

By the end of the Krebs cycle, all remaining carbons from the original glucose have been released as XX, all remaining hydrogens and electrons have been collected by XX and XX, and X ATP have been made. The major products per glucose are therefore XX, X NADH, X FADH₂, and X ATP. The big idea is that the Krebs cycle is mainly an electron-collection stage that prepares NADH and FADH₂ to deliver their electrons to theXX XX XX, where most ATP will be produced.

Krebs Cycle Enzymes You Need to Know

Per glucose, X Acetyl-CoA molecules enter the XX XX, giving a total of X carbons. During the cycle, those carbons are released as X CO₂, X ATP are made, and the remaining 8 hydrogens are removed and collected by electron carriers. The main enzyme numbers to know are X, X, X, and X.

Enzyme 3 is XX XX. It acts on XX. Because it is a dehydrogenase, it removes XX/electrons from the substrate and transfers them to XX, producing XX. This step also removes XX, so enzyme 3 performs both XX and XX

Enzyme 4 is X-XX dehydrogenase. It acts on X-XX. Like enzyme 3, it removes XX/electrons and produces XXX, while also removing XX. So enzyme 4 also performs an XX XX reaction.

Enzyme 6 is XX XX. It acts on XX and converts it into XX. It removes XX hydrogens from succinate, and those hydrogens/electrons are accepted by XX, producing XX. The naming pattern is simple: take the substrate name, succinate, and add dehydrogenase.

Enzyme 8 is XX XX. It acts on XX and converts it into XX. It removes XX/electrons from XX and transfers them to XX, producing XX.

A dehydrogenase removes XX/electrons from a substrate. Across enzymes 3, 4, 6, and 8, the remaining hydrogens from glucose are collected. Enzymes X, X, and X produce NADH, while enzyme X produces FADH₂.

Enzymes 3 and 4 are special because they do two things at the same time. They remove XX/electrons, which is oxidation, and they remove XX, which is XX. A reaction that does both is called XX XX.

The enzyme names to memorize are:

3 — XX XX
4 —X- XX dehydrogenase
6 — XX dehydrogenase
8 — XX dehydrogenase

The easiest naming rule is:

substrate name + dehydrogenase

So:

• isocitrate →XX XX

• α-ketoglutarate → X-XX dehydrogenase

• succinate → XX XX

• malate → XX XX

One extra detail from the slide: arsenic inhibits XX XX which is enzyme X

Krebs Cycle / Citric Acid Cycle

The Krebs cycle, also called the citric acid cycle or TCA cycle, happens in the mitochondrial matrix, which is the liquid part of the mitochondrion. The Acetyl-CoA formation step happens before this, as pyruvate moves from the cytoplasm into the mitochondrion. The Krebs cycle itself does not directly use oxygen, but it is still part of aerobic respiration because NADH and FADH₂ need the electron transport chain to be recycled, and that ultimately depends on oxygen.

The reason it is called the citric acid cycle is because citric acid is one of the first products made when Acetyl-CoA enters the cycle. It is also called the TCA cycle, which stands for tricarboxylic acid cycle, because citric acid is a tricarboxylic acid.

From one glucose molecule, you get 2 Acetyl-CoA molecules, and each Acetyl-CoA contains 2 carbons, so a total of 4 carbons enter the Krebs cycle. The overall products from these 2 Acetyl-CoA molecules are 4 CO₂, 6 NADH, 2 FADH₂, and 2 ATP. The slide also shows 6 H⁺ being released along with the 6 NADH.

The main purpose of the Krebs cycle is not to make a large amount of ATP directly. Instead, the main purpose is to finish breaking down the carbon skeleton and collect the remaining hydrogens and electrons. The cell wants to capture these electrons on the electron carriers NADH and FADH₂ so they can later be used to make much more ATP in the electron transport chain.

Glucose originally had 6 carbons. During Acetyl-CoA formation, 2 carbons were already removed as 2 CO₂, leaving 4 carbons to enter the Krebs cycle. During the Krebs cycle, these remaining 4 carbons are released as 4 CO₂. By the end of the Krebs cycle, all 6 original carbons from glucose have been released as carbon dioxide, so the original glucose has essentially been completely broken down and the CO₂ can be exhaled.

Glucose also originally had 12 hydrogens. During glycolysis, 2 hydrogens were removed, and during Acetyl-CoA formation, 2 more hydrogens were removed. That leaves 8 hydrogens to be collected during the Krebs cycle. By the end of the cycle, all 8 of these remaining hydrogens have been removed and their electrons have been captured by NADH and FADH₂.

During the Krebs cycle, NAD⁺ picks up hydrogen and electrons and becomes NADH. Per glucose, the Krebs cycle produces 6 NADH. Since NADH has gained electrons, it is in the reduced state. The cycle also uses another electron carrier called FAD, which accepts hydrogen and electrons to become FADH₂. Per glucose, the Krebs cycle produces 2 FADH₂. These two carriers are important because they will carry the electrons to the electron transport chain.

The Krebs cycle also makes 2 ATP total per glucose. These are only a small amount compared with the ATP that will be made later. The major ATP payoff comes during the electron transport chain and chemiosmosis.

When Acetyl-CoA enters the Krebs cycle, the CoA portion comes back off. Its job is mainly to carry the 2-carbon acetyl group into the cycle. The acetyl group continues through the cycle while CoA is released.

Decarboxylation also happens in the Krebs cycle. Decarboxylation means removing CO₂. There are 2 decarboxylation reactions per turn of the cycle, and because the cycle runs twice per glucose, this gives a total of 4 CO₂ released.

The Krebs cycle has 8 steps and 8 enzymes. You do not need to know all 8 enzymes, but you do need to know 4 of them.

By the end of the Krebs cycle, all remaining carbons from the original glucose have been released as CO₂, all remaining hydrogens and electrons have been collected by NADH and FADH₂, and 2 ATP have been made. The major products per glucose are therefore 4 CO₂, 6 NADH, 2 FADH₂, and 2 ATP. The big idea is that the Krebs cycle is mainly an electron-collection stage that prepares NADH and FADH₂ to deliver their electrons to the electron transport chain, where most ATP will be produced.

Krebs Cycle Enzymes You Need to Know

Per glucose, 2 Acetyl-CoA molecules enter the Krebs cycle, giving a total of 4 carbons. During the cycle, those carbons are released as 4 CO₂, 2 ATP are made, and the remaining 8 hydrogens are removed and collected by electron carriers. The main enzyme numbers to know are 3, 4, 6, and 8.

Enzyme 3 is isocitrate dehydrogenase. It acts on isocitrate. Because it is a dehydrogenase, it removes hydrogen/electrons from the substrate and transfers them to NAD⁺, producing NADH. This step also removes CO₂, so enzyme 3 performs both oxidation and decarboxylation.

Enzyme 4 is α-ketoglutarate dehydrogenase. It acts on α-ketoglutarate. Like enzyme 3, it removes hydrogen/electrons and produces NADH, while also removing CO₂. So enzyme 4 also performs an oxidative decarboxylation reaction.

Enzyme 6 is succinate dehydrogenase. It acts on succinate and converts it into fumarate. It removes two hydrogens from succinate, and those hydrogens/electrons are accepted by FAD, producing FADH₂. The naming pattern is simple: take the substrate name, succinate, and add dehydrogenase.

Enzyme 8 is malate dehydrogenase. It acts on malate and converts it into oxaloacetate. It removes hydrogen/electrons from malate and transfers them to NAD⁺, producing NADH.

A dehydrogenase removes hydrogen/electrons from a substrate. Across enzymes 3, 4, 6, and 8, the remaining hydrogens from glucose are collected. Enzymes 3, 4, and 8 produce NADH, while enzyme 6 produces FADH₂.

Enzymes 3 and 4 are special because they do two things at the same time. They remove hydrogen/electrons, which is oxidation, and they remove CO₂, which is decarboxylation. A reaction that does both is called oxidative decarboxylation.

The enzyme names to memorize are:

3 — Isocitrate dehydrogenase
4 — α-Ketoglutarate dehydrogenase
6 — Succinate dehydrogenase
8 — Malate dehydrogenase

The easiest naming rule is:

substrate name + dehydrogenase

So:

• isocitrate → isocitrate dehydrogenase

• α-ketoglutarate → α-ketoglutarate dehydrogenase

• succinate → succinate dehydrogenase

• malate → malate dehydrogenase

One extra detail from the slide: arsenic inhibits succinate dehydrogenase, which is enzyme 6

\membrane space for oxidative phosphorylation.

Electron Transport Chain / Chemiosmosis

The electron transport chain (ETC) is the final major stage of aerobic XX XX and is where the cell makes most of its XX. It takes place on the XX XX membrane, especially along the folds called the XX.

By the time the cell reaches this stage, XX and XX have already been removed from XX during glycolysis, XX-XX formation, and the XX XX. Those electrons are being carried by the reduced electron carriers XX and XX. When NADH and FADH₂ reach the inner mitochondrial membrane, they interact with a series of protein complexes in the XX XX XX and drop off their electrons.

As NADH and FADH₂ give up their electrons, they are XX back into XX and XX. The electrons are then passed from one XX XX to the next. Each successive carrier has a greater attraction for XX, so the electrons continue moving down the chain through a series of XX XX reactions.

The energy released as electrons move through the XX is used to pump XX ions from the mitochondrial matrix into the XX XX. Proton pumping occurs at several points in the chain, creating a XX concentration of XX in the intermembrane space and a lower concentration in the XX. This difference creates a proton electrochemical gradient, which stores XX XX.

The intermembrane space therefore has a XX pH because it contains a higher concentration of XX, while the mitochondrial matrix has a XX pH because it contains fewer X ions. The important idea is the XX XX between the two sides of the inner mitochondrial membrane.

The inner mitochondrial membrane is not freely permeable to X, so the hydrogen ions cannot simply diffuse back into the matrix anywhere they want. Their main pathway back across the membrane is through the enzyme XX XX. As H⁺ flows down its concentration gradient through ATP synthase, the enzyme uses that energy to combine XX + XX and make ATP.

This flow of H⁺ through ATP synthase is called XX. A good way to think about it is like water stored behind a dam. The H⁺ ions stored in the intermembrane space contain potential energy, and when they flow through ATP synthase, that stored energy is used to make XX.

The overall process of using electron transfer to create a proton gradient and then using that gradient to make ATP is called XX XX. It is called “oxidative” because XX are being transferred through a series of redox reactions, and it is called “phosphorylation” because a XX is added to ADP to form ATP.

XX is the final electron acceptor in the ETC. XX is the most electronegative molecule at the end of the pathway, so the electrons are ultimately transferred to XX. XX then combines with electrons and X to form water. This is why XX is essential for aerobic cellular respiration.

If oxygen is not available, the electrons have nowhere to go at the XX of the chain. The XX stops, the proton gradient cannot continue to be XX, ATP synthase cannot keep making XX through XX XX, and XX and XX cannot efficiently return to their oxidized forms.

The important sequence is:

NADH/FADH₂ deliver XX → electrons move through XX → energy pumps XX into XX XX → X gradient X → H⁺ flows through XX XX→ XX is made

The key locations are also important. The ETC and ATP synthase are in the XX XX XX, H⁺ accumulates in the XX space, and H⁺ flows back into the XX.

The main difference between the two parts of this stage is that the electron transport chain creates the X gradient, while chemiosmosis uses that gradient to make XX.

So the big picture is that the cell collected electrons from glucose onto XX and XX earlier in respiration. Those electrons now move down the XX toward oxygen, their energy is used to create an XX XX, and that gradient powers XX XX to make a large amount of ATP through XX XX.

Electron Transport Chain / Chemiosmosis

The electron transport chain (ETC) is the final major stage of aerobic cellular respiration and is where the cell makes most of its ATP. It takes place on the inner mitochondrial membrane, especially along the folds called the cristae.

By the time the cell reaches this stage, hydrogens and electrons have already been removed from glucose during glycolysis, Acetyl-CoA formation, and the Krebs cycle. Those electrons are being carried by the reduced electron carriers NADH and FADH₂. When NADH and FADH₂ reach the inner mitochondrial membrane, they interact with a series of protein complexes in the electron transport chain and drop off their electrons.

As NADH and FADH₂ give up their electrons, they are oxidized back into NAD⁺ and FAD. The electrons are then passed from one protein complex to the next. Each successive carrier has a greater attraction for electrons, so the electrons continue moving down the chain through a series of oxidation-reduction reactions.

The energy released as electrons move through the ETC is used to pump H⁺ ions from the mitochondrial matrix into the intermembrane space. Proton pumping occurs at several points in the chain, creating a high concentration of H⁺ in the intermembrane space and a lower concentration in the matrix. This difference creates a proton electrochemical gradient, which stores potential energy.

The intermembrane space therefore has a lower pH because it contains a higher concentration of H⁺, while the mitochondrial matrix has a higher pH because it contains fewer H⁺ ions. The important idea is the concentration difference between the two sides of the inner mitochondrial membrane.

The inner mitochondrial membrane is not freely permeable to H⁺, so the hydrogen ions cannot simply diffuse back into the matrix anywhere they want. Their main pathway back across the membrane is through the enzyme ATP synthase. As H⁺ flows down its concentration gradient through ATP synthase, the enzyme uses that energy to combine ADP + Pi and make ATP.

This flow of H⁺ through ATP synthase is called chemiosmosis. A good way to think about it is like water stored behind a dam. The H⁺ ions stored in the intermembrane space contain potential energy, and when they flow through ATP synthase, that stored energy is used to make ATP.

The overall process of using electron transfer to create a proton gradient and then using that gradient to make ATP is called oxidative phosphorylation. It is called “oxidative” because electrons are being transferred through a series of redox reactions, and it is called “phosphorylation” because a phosphate is added to ADP to form ATP.

Oxygen is the final electron acceptor in the ETC. Oxygen is the most electronegative molecule at the end of the pathway, so the electrons are ultimately transferred to oxygen. Oxygen then combines with electrons and H⁺ to form water. This is why oxygen is essential for aerobic cellular respiration.

If oxygen is not available, the electrons have nowhere to go at the end of the chain. The ETC stops, the proton gradient cannot continue to be maintained, ATP synthase cannot keep making ATP through oxidative phosphorylation, and NADH and FADH₂ cannot efficiently return to their oxidized forms.

The important sequence is:

NADH/FADH₂ deliver electrons → electrons move through ETC → energy pumps H⁺ into intermembrane space → H⁺ gradient forms → H⁺ flows through ATP synthase → ATP is made

The key locations are also important. The ETC and ATP synthase are in the inner mitochondrial membrane, H⁺ accumulates in the intermembrane space, and H⁺ flows back into the matrix.

The main difference between the two parts of this stage is that the electron transport chain creates the H⁺ gradient, while chemiosmosis uses that gradient to make ATP.

So the big picture is that the cell collected electrons from glucose onto NADH and FADH₂ earlier in respiration. Those electrons now move down the ETC toward oxygen, their energy is used to create an H⁺ gradient, and that gradient powers ATP synthase to make a large amount of ATP through oxidative phosphorylation.

Why Do We Need Oxygen to Live?

Oxygen is so important because it is highly XX , meaning it has a strong attraction for XX. In the electron transport chain, electrons are passed from one XX to another, and each carrier has a slightly greater XX for XX than the one before it. NADH donates electrons to XX XX, where carriers such as FMN (flavin mononucleotide) and iron-sulfur proteins pass the electrons along. This continues through the chain until the electrons finally reach XX, which has the strongest attraction for them and acts as the final XX XX.

As electrons move through the electron transport chain, their energy is used to pump XX ions into the intermembrane space. This maintains the proton gradient that is needed for XX XX. The H⁺ ions later flow back into the mitochondrial matrix through ATP synthase, and that flow powers the production of XX.

At the end of the chain, the electrons combine with XX and XX to form water. So oxygen is essentially the final “catcher’s mitt” for the electrons.

If oxygen is not available, XX cannot continue moving through the electron transport chain. If electron flow stops, X ions are no longer pumped into the intermembrane space. Without the H⁺ gradient, XX XX cannot keep producing ATP through oxidative phosphorylation. Since cells depend heavily on this ATP, prolonged loss of oxygen is incompatible with life.

The overall idea is:

Oxygen accepts XX → electron flow XX→ H⁺ gradient is XX → XX XX works → XX is produced

Arsenic

Arsenic can interfere with XX XX. One enzyme associated with this pathway is XX XX, an enzyme from the Krebs cycle that also functions as Complex II of the electron transport chain. Inhibition of enzymes involved in respiration disrupts the normal transfer of electrons and reduces the cell’s ability to produce XX.

The important point to remember is:

Arsenic can interfere with XX involved in XX XX, reducing XX XX.

Cyanide

Cyanide is extremely dangerous because it blocks XX XX of the electron transport chain by binding to cytochrome c oxidase.

Normally, Complex IV transfers electrons to oxygen.

If cyanide blocks XX:

• electrons cannot reach XX

• the electron transport chain XX

• XX pumping stops

• the H⁺ gradient XX

• ATP synthase can no longer make enough XX

This is why cyanide poisoning can be rapidly fatal even when oxygen is present in the blood: the cells cannot properly use the XX in the XX XX XX.

DNP

DNP makes the inner mitochondrial membrane, or cristae membrane, XX to XX XX.

Normally, the inner mitochondrial membrane does not allow X to freely XX. H⁺ is supposed to return to the matrix mainly through XX XX.

With DNP:

• X can leak directly back into the XX

• X bypasses XX XX

• the proton gradient is XX

• less XX is produced

This is called XX because electron transport can continue, but ATP production becomes separated, or uncoupled, from the proton gradient.

The lost energy is largely released as XX instead of being captured in XX.

Because the amount of H⁺ leakage can vary, XX does not simply reduce ATP production by one predictable fixed amount.

Main Idea

Oxygen is essential because it keeps the electron transport chain XX by serving as the final electron acceptor. Continuous electron flow maintains the X gradient, and that gradient powers XX XX.

So the sequence is:

NADH/FADH₂ donate XX → electrons move through XX → H⁺ is XX → H⁺ XX XX → ATP synthase makes XX → oxygen accepts the XX → XX forms

Without oxygen:

XX stops → X gradient disappears →XX XX stops → ATP production falls dramatically

Why Do We Need Oxygen to Live?

Oxygen is so important because it is highly electronegative, meaning it has a strong attraction for electrons. In the electron transport chain, electrons are passed from one carrier to another, and each carrier has a slightly greater attraction for electrons than the one before it. NADH donates electrons to Complex I, where carriers such as FMN (flavin mononucleotide) and iron-sulfur proteins pass the electrons along. This continues through the chain until the electrons finally reach oxygen, which has the strongest attraction for them and acts as the final electron acceptor.

As electrons move through the electron transport chain, their energy is used to pump H⁺ ions into the intermembrane space. This maintains the proton gradient that is needed for ATP production. The H⁺ ions later flow back into the mitochondrial matrix through ATP synthase, and that flow powers the production of ATP.

At the end of the chain, the electrons combine with H⁺ and oxygen to form water. So oxygen is essentially the final “catcher’s mitt” for the electrons.

If oxygen is not available, electrons cannot continue moving through the electron transport chain. If electron flow stops, H⁺ ions are no longer pumped into the intermembrane space. Without the H⁺ gradient, ATP synthase cannot keep producing ATP through oxidative phosphorylation. Since cells depend heavily on this ATP, prolonged loss of oxygen is incompatible with life.

The overall idea is:

Oxygen accepts electrons → electron flow continues → H⁺ gradient is maintained → ATP synthase works → ATP is produced

Arsenic

Arsenic can interfere with cellular respiration. One enzyme associated with this pathway is succinate dehydrogenase, an enzyme from the Krebs cycle that also functions as Complex II of the electron transport chain. Inhibition of enzymes involved in respiration disrupts the normal transfer of electrons and reduces the cell’s ability to produce ATP.

The important point to remember is:

Arsenic can interfere with enzymes involved in cellular respiration, reducing ATP production.

Cyanide

Cyanide is extremely dangerous because it blocks Complex IV of the electron transport chain by binding to cytochrome c oxidase.

Normally, Complex IV transfers electrons to oxygen.

If cyanide blocks Complex IV:

• electrons cannot reach oxygen

• the electron transport chain stops

• proton pumping stops

• the H⁺ gradient collapses

• ATP synthase can no longer make enough ATP

This is why cyanide poisoning can be rapidly fatal even when oxygen is present in the blood: the cells cannot properly use the oxygen in the electron transport chain.

DNP

DNP makes the inner mitochondrial membrane, or cristae membrane, leaky to H⁺ ions.

Normally, the inner mitochondrial membrane does not allow H⁺ to freely cross. H⁺ is supposed to return to the matrix mainly through ATP synthase.

With DNP:

• H⁺ can leak directly back into the matrix

• H⁺ bypasses ATP synthase

• the proton gradient is dissipated

• less ATP is produced

This is called uncoupling because electron transport can continue, but ATP production becomes separated, or uncoupled, from the proton gradient.

The lost energy is largely released as heat instead of being captured in ATP.

Because the amount of H⁺ leakage can vary, DNP does not simply reduce ATP production by one predictable fixed amount.

Main Idea

Oxygen is essential because it keeps the electron transport chain moving by serving as the final electron acceptor. Continuous electron flow maintains the H⁺ gradient, and that gradient powers ATP synthase.

So the sequence is:

NADH/FADH₂ donate electrons → electrons move through ETC → H⁺ is pumped → H⁺ gradient forms → ATP synthase makes ATP → oxygen accepts the electrons → water forms

Without oxygen:

ETC stops → H⁺ gradient disappears → ATP synthase stops → ATP production falls dramatically

Two Ways to Make ATP

There are two main ways ATP is made during cellular respiration: XX XX phosphorylation and XX XX. Both ultimately make XX from XX + XX, but they do it in very different ways.

Substrate-Level Phosphorylation

Substrate-level phosphorylation makes XX by transferring a XX XX directly from a XX XX to XX. It does not require ATP synthase or a hydrogen ion gradient.

This happens during XX and the XX. A phosphorylated molecule has a XX XX that an enzyme can transfer directly onto XX, forming XX.

An important example is XX XX during glycolysis. Pyruvate kinase transfers the phosphate from XX (phosphoenolpyruvate) directly to XX. This produces XX and XX.

So the basic idea is:

Phosphorylated substrate + ADP → product + ATP

The important thing to remember is that the phosphate comes directly from another molecule.

Oxidative Phosphorylation

Oxidative phosphorylation makes XX using the XX XX XX, an X gradient, and XX XX. This is where most XX from aerobic cellular respiration is produced.

NADH and FADH₂ bring XX-XX electrons to the electron transport chain. As the electrons move through a series of oxidation-reduction reactions, their energy is used to pump X into the XX XX.

This creates an X XX . The H⁺ ions then flow back into the XX XX through. The movement of H⁺ through ATP synthase provides the energy needed to combine:

XX + XX → XX

So oxidative phosphorylation indirectly uses the XX originally stored in food to make XX.

How ATP Synthase Works

ATP synthase is an XX XX XX in the inner mitochondrial membrane. XX XX flow through part of the protein because they are moving down their electrochemical gradient.

As H⁺ flows through ATP synthase, the XX XX. A stator holds part of the enzyme stationary, while a central stalk rotates. This rotation changes the shape of catalytic sites in the enzyme.

The catalytic sites then bring XX and XX XXX (Pi) together to form XX.

ATP synthase can be thought of like a water wheel or turbine. As long as H⁺ continues flowing through it, the XX can continue producingXX.

Why Food Is Important

The hydrogen ions and electrons ultimately come from the food being broken down during cellular respiration. If there is no fuel being broken down, fewer NADH and FADH₂ molecules are produced, electron transport cannot be maintained, and the H⁺ gradient cannot continue powering ATP synthase.

Main Difference

Substrate-level phosphorylation:

• phosphate is transferred directly from a XX to XX

• does not use XX XX

• does not require an X XX

• occurs in XX and the XX XX

• produces a relatively XX XX of XX

Oxidative phosphorylation:

• depends on XX-XX reactions

• uses the XX XX XX

• creates an X XX

• H⁺ flows through XX XX

• occurs at the XX XX XX

• produces most of the XX in aerobic cellular respiration

The easiest way to remember the difference is:

Substrate-level = XX goes directly ontoXX

Oxidative = XX → X XX→ XX XX → XX

Two Ways to Make ATP

There are two main ways ATP is made during cellular respiration: substrate-level phosphorylation and oxidative phosphorylation. Both ultimately make ATP from ADP + phosphate, but they do it in very different ways.

Substrate-Level Phosphorylation

Substrate-level phosphorylation makes ATP by transferring a phosphate group directly from a phosphorylated substrate to ADP. It does not require ATP synthase or a hydrogen ion gradient.

This happens during glycolysis and the Krebs cycle. A phosphorylated molecule has a phosphate group that an enzyme can transfer directly onto ADP, forming ATP.

An important example is pyruvate kinase during glycolysis. Pyruvate kinase transfers the phosphate from PEP (phosphoenolpyruvate) directly to ADP. This produces ATP and pyruvate.

So the basic idea is:

Phosphorylated substrate + ADP → product + ATP

The important thing to remember is that the phosphate comes directly from another molecule.

Oxidative Phosphorylation

Oxidative phosphorylation makes ATP using the electron transport chain, an H⁺ gradient, and ATP synthase. This is where most ATP from aerobic cellular respiration is produced.

NADH and FADH₂ bring high-energy electrons to the electron transport chain. As the electrons move through a series of oxidation-reduction reactions, their energy is used to pump H⁺ into the intermembrane space.

This creates an H⁺ gradient. The H⁺ ions then flow back into the mitochondrial matrix through ATP synthase. The movement of H⁺ through ATP synthase provides the energy needed to combine:

ADP + Pi → ATP

So oxidative phosphorylation indirectly uses the energy originally stored in food to make ATP.

How ATP Synthase Works

ATP synthase is an integral membrane protein in the inner mitochondrial membrane. H⁺ ions flow through part of the protein because they are moving down their electrochemical gradient.

As H⁺ flows through ATP synthase, the rotor spins. A stator holds part of the enzyme stationary, while a central stalk rotates. This rotation changes the shape of catalytic sites in the enzyme.

The catalytic sites then bring ADP and inorganic phosphate (Pi) together to form ATP.

ATP synthase can be thought of like a water wheel or turbine. As long as H⁺ continues flowing through it, the enzyme can continue producing ATP.

Why Food Is Important

The hydrogen ions and electrons ultimately come from the food being broken down during cellular respiration. If there is no fuel being broken down, fewer NADH and FADH₂ molecules are produced, electron transport cannot be maintained, and the H⁺ gradient cannot continue powering ATP synthase.

Main Difference

Substrate-level phosphorylation:

• phosphate is transferred directly from a substrate to ADP

• does not use ATP synthase

• does not require an H⁺ gradient

• occurs in glycolysis and the Krebs cycle

• produces a relatively small amount of ATP

Oxidative phosphorylation:

• depends on oxidation-reduction reactions

• uses the electron transport chain

• creates an H⁺ gradient

• H⁺ flows through ATP synthase

• occurs at the inner mitochondrial membrane

• produces most of the ATP in aerobic cellular respiration

The easiest way to remember the difference is:

Substrate-level = phosphate goes directly onto ADP

Oxidative = electrons → H⁺ gradient → ATP synthase → ATP

When O₂ Is Limited

After glycolysis, glucose has been broken down into pyruvate. Pyruvate is an important junction point because what happens next depends on whether oxygen is available.

If oxygen is present, pyruvate enters the XX, is converted into XX XX, and then enters the citric acid/XX XX as part of aerobic XX XX

If oxygen is absent or limited, the cell can use XX instead. Fermentation allows cells to keep making a small amount of XX through glycolysis even when oxygen is XX.

Fermentation does not always produce the same XX XX. Depending on the organism or cell type, pyruvate can be converted into products such as XX, XX, or other fermentation products.

In humans, pyruvate can be converted into XX This is also important in pyruvate dehydrogenase complex deficiency (PDCD), or congenital infantile lactic acidosis, because pyruvate cannot efficiently enter the Acetyl-CoA pathway and more pyruvate can be directed toward lactate production.

The main idea is:

O₂ present → XX → XX-XX→ XX XX

O₂ limited/absent → XX → XX → lactate, ethanol, or other products

 Fermentation: Making ATP without XX

Animal cells reduce XX to XX (lactic

acid) …causes a burning sensation in muscles.

Climb the stairs in SL and try it!

Yeast & bacteria decarboxylate pyruvate first,

then reduce acetaldehyde, regenerating NAD+

and producing alcohol as a byproduct.

Glycolysis does not require O2, and yields 2

ATPs.

Glycolysis, however, DOES require NAD+, and if

not regenerated, can no longer make ATP.

When O₂ Is Limited

After glycolysis, glucose has been broken down into pyruvate. Pyruvate is an important junction point because what happens next depends on whether oxygen is available.

If oxygen is present, pyruvate enters the mitochondrion, is converted into Acetyl-CoA, and then enters the citric acid/Krebs cycle as part of aerobic cellular respiration.

If oxygen is absent or limited, the cell can use fermentation instead. Fermentation allows cells to keep making a small amount of ATP through glycolysis even when oxygen is unavailable.

Fermentation does not always produce the same end product. Depending on the organism or cell type, pyruvate can be converted into products such as lactate, ethanol, or other fermentation products.

In humans, pyruvate can be converted into lactate. This is also important in pyruvate dehydrogenase complex deficiency (PDCD), or congenital infantile lactic acidosis, because pyruvate cannot efficiently enter the Acetyl-CoA pathway and more pyruvate can be directed toward lactate production.

The main idea is:

O₂ present → Pyruvate → Acetyl-CoA → Krebs cycle

O₂ limited/absent → Pyruvate → Fermentation → lactate, ethanol, or other products

 Fermentation: Making ATP without O2

Animal cells reduce pyruvate to lactate (lactic

acid) …causes a burning sensation in muscles.

Climb the stairs in SL and try it!

Yeast & bacteria decarboxylate pyruvate first,

then reduce acetaldehyde, regenerating NAD+

and producing alcohol as a byproduct.

Glycolysis does not require O2, and yields 2

ATPs.

Glycolysis, however, DOES require NAD+, and if

not regenerated, can no longer make ATP.

Fermentation: Making ATP Without O₂

Glycolysis does not require XX and produces a net of X XX per glucose. However, glycolysis does require XX. If XX is not regenerated, glycolysis eventually stops because the cell runs out of oxidized XX to accept electrons.

The purpose of fermentation is mainly to regenerate XX so glycolysis can keep running and continue making a small amount of XX when XX is absent.

During glycolysis, XX accepts electrons and hydrogen and becomes XX. Without oxygen, the electron transport chain cannot efficiently oxidize XX back to XX. Fermentation solves this problem by transferring the hydrogens/electrons from XX onto another molecule.

In animal cells, pyruvate acts as a temporary place to put those hydrogens. NADH transfers its hydrogens/electrons to XX, which converts pyruvate into XX (lactic acid). At the same time, XX is converted back into XX. The regenerated NAD⁺ can then return to glycolysis and help produce more ATP.

So in animal cells:

XX+ XX →XX + XX

The important idea is not that fermentation makes extra ATP directly. The ATP still comes from glycolysis. Fermentation simply regenerates XX so glycolysis can continue.

Yeast and some bacteria use a different type of fermentation called XX XX. Pyruvate is first decarboxylated, meaning XX is removed. This forms acetaldehyde. NADH then transfers its hydrogens/electrons to acetaldehyde, producing ethanol and regenerating NAD⁺.

So in yeast:

Pyruvate → Acetaldehyde + CO₂

Then:

Acetaldehyde + NADH → Ethanol + NAD⁺

This is why yeast growing with sugar but little or no oxygen can produce ethanol, which is used in products such as beer and wine.

The main purpose of both types of fermentation is the same:

regenerate NAD⁺ so glycolysis can continue

The big difference is the final product:

• Animal cells: XX

• Yeast/some bacteria:XX + XX

The key idea to remember is:

No O₂ → ETC cannot regenerate enough NAD⁺ → fermentation regenerates NAD⁺ → glycolysis continues → 2 ATP can still be made

Fermentation: Making ATP Without O₂

Glycolysis does not require oxygen and produces a net of 2 ATP per glucose. However, glycolysis does require NAD⁺. If NAD⁺ is not regenerated, glycolysis eventually stops because the cell runs out of oxidized NAD⁺ to accept electrons.

The purpose of fermentation is mainly to regenerate NAD⁺ so glycolysis can keep running and continue making a small amount of ATP when oxygen is absent.

During glycolysis, NAD⁺ accepts electrons and hydrogen and becomes NADH. Without oxygen, the electron transport chain cannot efficiently oxidize NADH back to NAD⁺. Fermentation solves this problem by transferring the hydrogens/electrons from NADH onto another molecule.

In animal cells, pyruvate acts as a temporary place to put those hydrogens. NADH transfers its hydrogens/electrons to pyruvate, which converts pyruvate into lactate (lactic acid). At the same time, NADH is converted back into NAD⁺. The regenerated NAD⁺ can then return to glycolysis and help produce more ATP.

So in animal cells:

Pyruvate + NADH → Lactate + NAD⁺

The important idea is not that fermentation makes extra ATP directly. The ATP still comes from glycolysis. Fermentation simply regenerates NAD⁺ so glycolysis can continue.

Yeast and some bacteria use a different type of fermentation called alcoholic fermentation. Pyruvate is first decarboxylated, meaning CO₂ is removed. This forms acetaldehyde. NADH then transfers its hydrogens/electrons to acetaldehyde, producing ethanol and regenerating NAD⁺.

So in yeast:

Pyruvate → Acetaldehyde + CO₂

Then:

Acetaldehyde + NADH → Ethanol + NAD⁺

This is why yeast growing with sugar but little or no oxygen can produce ethanol, which is used in products such as beer and wine.

The main purpose of both types of fermentation is the same:

regenerate NAD⁺ so glycolysis can continue

The big difference is the final product:

• Animal cells: lactate

• Yeast/some bacteria: ethanol + CO₂

The key idea to remember is:

No O₂ → ETC cannot regenerate enough NAD⁺ → fermentation regenerates NAD⁺ → glycolysis continues → 2 ATP can still be made

Other Macromolecules Can Also Be Used to Make ATP

Cellular respiration does not only use XX. Proteins and fats can also be broken down and fed into the cellular respiration pathway. They simply enter at different XX depending on what type of molecule they are.

Proteins are first broken down into XX XX. Before an amino acid can be used for energy, its amino group has to be XX. This process is called XX. The removed nitrogen can form XX, which is converted to urea and excreted in urine. The remaining carbon skeleton can then enter XX XX at different points depending on the amino acid and its R group. It may enter near the end of glycolysis, be converted into Acetyl-CoA, or enter somewhere in the Krebs cycle.

Fats are an especially XX XX source because they contain many carbon-hydrogen bonds and therefore store a lot of potential energy. The slide notes that fats have about twice as many C–H bonds as glucose and, carbon-for-carbon, can produce about 20% more ATP than glucose.

Fats are broken into XX andXX XX. The glycerol portion can be phosphorylated and converted into XX, allowing it to enter glycolysis.

The fatty acids are broken down by a process called XX XX. Beta oxidation cuts fatty acids into small X-XX pieces, which are converted into XX XX. Acetyl-CoA can then enter the XX XX and continue through XX XX to make XX.

So the important pathways are:

Carbohydrates → XX → XX

Proteins → XX XX → XX → glycolysis / Acetyl-CoA / Krebs cycle

Fats → XXl → XX → glycolysis

Fats → XX XX → XX XX → XX XX → XX XX

The main idea is that carbohydrates, proteins, and fats can all ultimately be used as fuel for ATP production, but they enter cellular respiration at different points.

Other Macromolecules Can Also Be Used to Make ATP

Cellular respiration does not only use carbohydrates. Proteins and fats can also be broken down and fed into the cellular respiration pathway. They simply enter at different points depending on what type of molecule they are.

Proteins are first broken down into amino acids. Before an amino acid can be used for energy, its amino group has to be removed. This process is called deamination. The removed nitrogen can form ammonia, which is converted to urea and excreted in urine. The remaining carbon skeleton can then enter cellular respiration at different points depending on the amino acid and its R group. It may enter near the end of glycolysis, be converted into Acetyl-CoA, or enter somewhere in the Krebs cycle.

Fats are an especially good energy source because they contain many carbon-hydrogen bonds and therefore store a lot of potential energy. The slide notes that fats have about twice as many C–H bonds as glucose and, carbon-for-carbon, can produce about 20% more ATP than glucose.

Fats are broken into glycerol and fatty acids. The glycerol portion can be phosphorylated and converted into G3P, allowing it to enter glycolysis.

The fatty acids are broken down by a process called β-oxidation. Beta oxidation cuts fatty acids into small 2-carbon pieces, which are converted into Acetyl-CoA. Acetyl-CoA can then enter the Krebs cycle and continue through cellular respiration to make ATP.

So the important pathways are:

Carbohydrates → sugars → glycolysis

Proteins → amino acids → deamination → glycolysis / Acetyl-CoA / Krebs cycle

Fats → glycerol → G3P → glycolysis

Fats → fatty acids → β-oxidation → Acetyl-CoA → Krebs cycle

The main idea is that carbohydrates, proteins, and fats can all ultimately be used as fuel for ATP production, but they enter cellular respiration at different points.

Bookkeeping: Tracking ATP, NADH, and FADH₂

When tracking cellular respiration, it helps to separate the ATP made directly by substrate-level phosphorylation from the ATP made later by oxidative phosphorylation.

During glycolysis, the cell makes X net ATP and produces X NADH. These NADH contain the electrons that were removed from glucose. Because glycolysis happens in the cytoplasm, those electrons have to be shuttled into the XX. Depending on the type of electron shuttle used, the electrons may enter the mitochondrion as the equivalent of 2 XX or 2 XX.

During Acetyl-CoA formation (pyruvate oxidation), there is no XX made directly. However, the two pyruvate molecules are converted into X Acetyl-CoA, X CO₂ are released, and X NADH are produced.

During the Krebs cycle, the cell makes X ATP directly. It also produces X NADH and X FADH₂, which carry XX XX electrons to the electron transport chain. The Krebs cycle also releases the remaining XX , meaning that all 6 carbons from the original glucose have now been released as carbon dioxide.

Before oxidative phosphorylation begins, the cell has therefore made a total of:

X ATP from glycolysis + X ATP from Acetyl-CoA formation + X ATP from Krebs = 4 ATP directly

These 4 ATP are made by XX XX XX

The electron carriers produced per glucose are approximately:

X NADH from glycolysis
X NADH from Acetyl-CoA formation
X NADH from Krebs cycle
XFADH₂ from Krebs cycle

So the cell has the reducing power of about 10 NADH + 2 FADH₂, although the 2 cytoplasmic NADH from glycolysis may enter the mitochondrion differently depending on the shuttle used.

During oxidative phosphorylation, NADH and FADH₂ donate their electrons to the XX XX. A NADH generally contributes about XX ATP, while a FADH₂ contributes about XX ATP. These are approximate values, which is why the ATP total is given as a range rather than one exact number.

Oxidative phosphorylation produces about:

XX-XX ATP

Adding the 4 ATP made directly gives a maximum total of approximately:

XX-XX ATP per glucose

The reason the total may be 30 or 32 ATP is mainly because the NADH produced during glycolysis is outside the mitochondrion and its electrons have to be transferred inside using an electron shuttle. Different shuttles deliver those electrons at different energy levels.

Main numbers to know

Glycolysis: X ATP + X NADH
Acetyl-CoA formation: X ATP + X NADH
Krebs cycle: X ATP + X NADH + X FADH₂
Oxidative phosphorylation: about XX–XX ATP

Total per glucose: about XX–XX ATP

The big idea is that only 4 ATP are made directly, while the majority of ATP comes later from the electrons carried by NADH and FADH₂ into oxidative phosphorylation.

Bookkeeping: Tracking ATP, NADH, and FADH₂

When tracking cellular respiration, it helps to separate the ATP made directly by substrate-level phosphorylation from the ATP made later by oxidative phosphorylation.

During glycolysis, the cell makes 2 net ATP and produces 2 NADH. These NADH contain the electrons that were removed from glucose. Because glycolysis happens in the cytoplasm, those electrons have to be shuttled into the mitochondrion. Depending on the type of electron shuttle used, the electrons may enter the mitochondrion as the equivalent of 2 NADH or 2 FADH₂.

During Acetyl-CoA formation (pyruvate oxidation), there is no ATP made directly. However, the two pyruvate molecules are converted into 2 Acetyl-CoA, 2 CO₂ are released, and 2 NADH are produced.

During the Krebs cycle, the cell makes 2 ATP directly. It also produces 6 NADH and 2 FADH₂, which carry high-energy electrons to the electron transport chain. The Krebs cycle also releases the remaining 4 CO₂, meaning that all 6 carbons from the original glucose have now been released as carbon dioxide.

Before oxidative phosphorylation begins, the cell has therefore made a total of:

2 ATP from glycolysis + 0 ATP from Acetyl-CoA formation + 2 ATP from Krebs = 4 ATP directly

These 4 ATP are made by substrate-level phosphorylation.

The electron carriers produced per glucose are approximately:

2 NADH from glycolysis
2 NADH from Acetyl-CoA formation
6 NADH from Krebs cycle
2 FADH₂ from Krebs cycle

So the cell has the reducing power of about 10 NADH + 2 FADH₂, although the 2 cytoplasmic NADH from glycolysis may enter the mitochondrion differently depending on the shuttle used.

During oxidative phosphorylation, NADH and FADH₂ donate their electrons to the electron transport chain. A NADH generally contributes about 2.5 ATP, while a FADH₂ contributes about 1.5 ATP. These are approximate values, which is why the ATP total is given as a range rather than one exact number.

Oxidative phosphorylation produces about:

26–28 ATP

Adding the 4 ATP made directly gives a maximum total of approximately:

30–32 ATP per glucose

The reason the total may be 30 or 32 ATP is mainly because the NADH produced during glycolysis is outside the mitochondrion and its electrons have to be transferred inside using an electron shuttle. Different shuttles deliver those electrons at different energy levels.

Main numbers to know

Glycolysis: 2 ATP + 2 NADH
Acetyl-CoA formation: 0 ATP + 2 NADH
Krebs cycle: 2 ATP + 6 NADH + 2 FADH₂
Oxidative phosphorylation: about 26–28 ATP

Total per glucose: about 30–32 ATP

The big idea is that only 4 ATP are made directly, while the majority of ATP comes later from the electrons carried by NADH and FADH₂ into oxidative phosphorylation.

Chapter 10 photosynthesis:

Photosynthesis is carried out by several different groups of organisms, not just plants. Plants are one of the XX and most familiar groups that perform photosynthesis. Multicellular algae also photosynthesize, but algae are not plants and generally live in aquatic environments. Many protists are also photosynthetic.

Photosynthesis originally evolved in XX XX, especially cyanobacteria, sometimes called blue-green bacteria. Cyanobacteria use photosynthetic pigments and were very important in the evolution of oxygen-producing photosynthesis.

Not all photosynthetic organisms are XX. Green pigments such as chlorophyll are very common, but other photosynthetic pigments can make organisms appear purple, red, orange, or other colors. For example, some bacteria such as purple sulfur bacteria use different pigments for photosynthesis.

XX are not photosynthetic. They do not capture light energy to make their own sugars.

The main functions of photosynthesis are to collect XX XX, convert that XX XX into XX XX, and then store the chemical energy in a stable form such as XX.

So the major photosynthetic groups to remember are plants, algae, photosynthetic protists, cyanobacteria, and other photosynthetic bacteria.

Chapter 10 photosynthesis:

Photosynthesis is carried out by several different groups of organisms, not just plants. Plants are one of the largest and most familiar groups that perform photosynthesis. Multicellular algae also photosynthesize, but algae are not plants and generally live in aquatic environments. Many protists are also photosynthetic.

Photosynthesis originally evolved in photosynthetic bacteria, especially cyanobacteria, sometimes called blue-green bacteria. Cyanobacteria use photosynthetic pigments and were very important in the evolution of oxygen-producing photosynthesis.

Not all photosynthetic organisms are green. Green pigments such as chlorophyll are very common, but other photosynthetic pigments can make organisms appear purple, red, orange, or other colors. For example, some bacteria such as purple sulfur bacteria use different pigments for photosynthesis.

Fungi are not photosynthetic. They do not capture light energy to make their own sugars.

The main functions of photosynthesis are to collect solar energy, convert that light energy into chemical energy, and then store the chemical energy in a stable form such as sugar.

So the major photosynthetic groups to remember are plants, algae, photosynthetic protists, cyanobacteria, and other photosynthetic bacteria.

Structure of Chlorophyll

Chlorophyll is the main pigment used to capture XX XX during photosynthesis. Its job is basically to act like a solar collector by absorbing specific XX of light.

Chlorophyll appears XX because it does not absorb green light very well. Instead, green light is mostly XX back to our eyes. Chlorophyll strongly absorbs XX XX and XX XX, which are especially useful for photosynthesis.

The chlorophyll molecule has a XX ring that contains a magnesium (Mg) atom. This ring is the part of the molecule that actually absorbs light energy.

Chlorophyll also has a long XX side chain, or tail. This tail is made mostly of XX and XX, so it is very XX. Its main job is to anchor the XX XX into the XX XX

There are two important types of chlorophyll:

Chlorophyll a

• the major XX of photosynthesis

• usually the XX chlorophyll

• directly involved in the main photosynthetic reactions

Chlorophyll b

• an XX XX

• helps absorb additional wavelengths of light

• has a slightly different chemical group, which changes the wavelengths it absorbs slightly

Chlorophyll a has one less XX than chlorophyll b, so chlorophyll a is slightly lessXX

Other photosynthetic pigments also help capture light. Carotenoids appear orange, while xanthophylls appear yellow. These accessory pigments absorb wavelengths that chlorophyll does not absorb as strongly and help broaden the range of light that can be used for photosynthesis.

The main idea is:

Chlorophyll absorbs blue and red light, reflects green light, and uses its ring structure to capture solar energy, while its hydrophobic tail anchors it in the thylakoid membrane.

Structure of Chlorophyll

Chlorophyll is the main pigment used to capture light energy during photosynthesis. Its job is basically to act like a solar collector by absorbing specific wavelengths of light.

Chlorophyll appears green because it does not absorb green light very well. Instead, green light is mostly reflected back to our eyes. Chlorophyll strongly absorbs blue light and red light, which are especially useful for photosynthesis.

The chlorophyll molecule has a porphyrin ring that contains a magnesium (Mg) atom. This ring is the part of the molecule that actually absorbs light energy.

Chlorophyll also has a long hydrocarbon side chain, or tail. This tail is made mostly of carbon and hydrogen, so it is very hydrophobic. Its main job is to anchor the chlorophyll molecule into the thylakoid membrane.

There are two important types of chlorophyll:

Chlorophyll a

• the major pigment of photosynthesis

• usually the predominant chlorophyll

• directly involved in the main photosynthetic reactions

Chlorophyll b

• an accessory pigment

• helps absorb additional wavelengths of light

• has a slightly different chemical group, which changes the wavelengths it absorbs slightly

Chlorophyll a has one less oxygen than chlorophyll b, so chlorophyll a is slightly less polar.

Other photosynthetic pigments also help capture light. Carotenoids appear orange, while xanthophylls appear yellow. These accessory pigments absorb wavelengths that chlorophyll does not absorb as strongly and help broaden the range of light that can be used for photosynthesis.

The main idea is:

Chlorophyll absorbs blue and red light, reflects green light, and uses its ring structure to capture solar energy, while its hydrophobic tail anchors it in the thylakoid membrane.

Why Leaves Are Green: Interaction of Light with Chloroplasts

Leaves look green because of how XX interacts with XX inside XXs. Some wavelengths of light are absorbed, some are transmitted, and green light is mostly reflected back to our eyes, which is why leaves appear green.

A chloroplast has an XX, an XX XX, and an iXX XX, similar to a mitochondrion. However, unlike the mitochondrial intermembrane space, the chloroplast intermembrane space does not play the major role in XX XX

The XX is the liquid-filled part of the chloroplast. It is similar to the mitochondrial matrix. The Calvin cycle takes place in the XX, and the chloroplast’s own DNA/genome is also found there.

Inside the chloroplast are flattened membrane sacs called XX. The thylakoid membrane is packed with XX and other photosynthetic pigments, which is why these membranes appear green and are able to absorb light energy.

Thylakoids are arranged in stacks. A stack of thylakoids is called a:

XX

So:

Thylakoid = XX XX sac

Granum = stack of XX

The inside space of a thylakoid is called the:

XX XX

The thylakoid lumen is especially important because X ions accumulate there during the light reactions. This creates a proton gradient.

The H⁺ ions then flow back across the thylakoid membrane through XX XX, allowing the chloroplast to make ATP by chemiosmosis, similar to how mitochondria use an H⁺ gradient to make ATP.

The key locations to remember are:

XX→ Calvin cycle

Thylakoid membrane →XX + XX

Thylakoid lumen →X XX

Granum → stack of XX

The big idea is that chloroplasts absorb light using chlorophyll in the thylakoid membranes, reflect green light, and use the resulting H⁺ gradient to help produce ATP for photosynthesis

Why Leaves Are Green: Interaction of Light with Chloroplasts

Leaves look green because of how light interacts with chlorophyll inside chloroplasts. Some wavelengths of light are absorbed, some are transmitted, and green light is mostly reflected back to our eyes, which is why leaves appear green.

A chloroplast has an outer membrane, an inner membrane, and an intermembrane space, similar to a mitochondrion. However, unlike the mitochondrial intermembrane space, the chloroplast intermembrane space does not play the major role in ATP production.

The stroma is the liquid-filled part of the chloroplast. It is similar to the mitochondrial matrix. The Calvin cycle takes place in the stroma, and the chloroplast’s own DNA/genome is also found there.

Inside the chloroplast are flattened membrane sacs called thylakoids. The thylakoid membrane is packed with chlorophyll and other photosynthetic pigments, which is why these membranes appear green and are able to absorb light energy.

Thylakoids are arranged in stacks. A stack of thylakoids is called a:

granum

So:

Thylakoid = one membrane sac

Granum = stack of thylakoids

The inside space of a thylakoid is called the:

thylakoid lumen

The thylakoid lumen is especially important because H⁺ ions accumulate there during the light reactions. This creates a proton gradient.

The H⁺ ions then flow back across the thylakoid membrane through ATP synthase, allowing the chloroplast to make ATP by chemiosmosis, similar to how mitochondria use an H⁺ gradient to make ATP.

The key locations to remember are:

Stroma → Calvin cycle

Thylakoid membrane → chlorophyll + light reactions

Thylakoid lumen → H⁺ accumulation

Granum → stack of thylakoids

The big idea is that chloroplasts absorb light using chlorophyll in the thylakoid membranes, reflect green light, and use the resulting H⁺ gradient to help produce ATP for photosynthesis

Electromagnetic Spectrum: Light and Electrons

Light is made of discrete packets of energy called XX. Different photons carry different amounts of energy depending on their wavelength. Shorter wavelengths have XX energy, while longer wavelengths haveXX energy.

Within visible light, XX and XX light have shorter wavelengths and therefore more energy, while XX ight has a longer wavelength and less energy.

When light strikes chlorophyll, XX can be absorbed by electrons in the chlorophyll molecule. When an electron absorbs the energy from a photon, it becomes XX and moves from a lower energy level to a XX XX XX.

Once an electron is excited, two things can happen. It may fall back down to its XX, XX-energy or ground state. When this happens, the absorbed energy is released as XX or fluorescence.

The other possibility is what is important for photosynthesis. If the excited electron gains enough energy, it can be transferred away from chlorophyll and captured by an XX XX XX. This transfer of an excited electron is what begins the electron-transfer reactions involved in photosynthesis.

The main idea is:

Photon absorbed → electron becomes excited → electron either falls back and releases energy, or is captured by an electron acceptor and enters photosynthesis

Also remember:

Short wavelength = higher energy

Long wavelength = lower energy

Electromagnetic Spectrum: Light and Electrons

Light is made of discrete packets of energy called photons. Different photons carry different amounts of energy depending on their wavelength. Shorter wavelengths have higher energy, while longer wavelengths have lower energy.

Within visible light, violet and blue light have shorter wavelengths and therefore more energy, while red light has a longer wavelength and less energy.

When light strikes chlorophyll, photons can be absorbed by electrons in the chlorophyll molecule. When an electron absorbs the energy from a photon, it becomes excited and moves from a lower energy level to a higher energy level.

Once an electron is excited, two things can happen. It may fall back down to its original, lower-energy or ground state. When this happens, the absorbed energy is released as heat or fluorescence.

The other possibility is what is important for photosynthesis. If the excited electron gains enough energy, it can be transferred away from chlorophyll and captured by an electron acceptor molecule. This transfer of an excited electron is what begins the electron-transfer reactions involved in photosynthesis.

The main idea is:

Photon absorbed → electron becomes excited → electron either falls back and releases energy, or is captured by an electron acceptor and enters photosynthesis

Also remember:

Short wavelength = higher energy

Long wavelength = lower energy

Three Classic Experiments: Which Wavelengths Matter for Photosynthesis?

These experiments were designed to answer one main question: which wavelengths of light are most important for photosynthesis?

The absorption spectrum shows which wavelengths are absorbed by photosynthetic pigments. Chlorophyll absorbs light most strongly in the XX/XX region and also strongly in the XX XX. The tall peaks on the graph mean that a lot of light is being absorbed at those wavelengths. Very little light is absorbed in the XX and yellow regions, which is why green light is mostly reflected instead of used.

The action spectrum shows which wavelengths actually drive photosynthesis most effectively. The rate of photosynthesis is highest in the XX-XX region and again in the red/far-red region. XX is much lower in the green region.

The Engelmann experiment used a photosynthetic alga and aerobic bacteria. The bacteria were attracted to areas with the highest oxygen concentration because they use oxygen. The bacteria gathered mostly around the parts of the alga exposed to blue/violet light and red light, showing that those wavelengths caused the greatest oxygen production and therefore the greatest rate of photosynthesis.

The main conclusion is:

XX XX and XX light are the most effective wavelengths for driving photosynthesis.

XX XX is much less effective because chlorophyll absorbs very little of it. That is also why green light can be used as a safelight when working with photosynthetic organisms, because it interferes less with photosynthesis

Three Classic Experiments: Which Wavelengths Matter for Photosynthesis?

These experiments were designed to answer one main question: which wavelengths of light are most important for photosynthesis?

The absorption spectrum shows which wavelengths are absorbed by photosynthetic pigments. Chlorophyll absorbs light most strongly in the blue/violet region and also strongly in the red region. The tall peaks on the graph mean that a lot of light is being absorbed at those wavelengths. Very little light is absorbed in the green and yellow regions, which is why green light is mostly reflected instead of used.

The action spectrum shows which wavelengths actually drive photosynthesis most effectively. The rate of photosynthesis is highest in the violet-blue region and again in the red/far-red region. Photosynthesis is much lower in the green region.

The Engelmann experiment used a photosynthetic alga and aerobic bacteria. The bacteria were attracted to areas with the highest oxygen concentration because they use oxygen. The bacteria gathered mostly around the parts of the alga exposed to blue/violet light and red light, showing that those wavelengths caused the greatest oxygen production and therefore the greatest rate of photosynthesis.

The main conclusion is:

Violet-blue and red light are the most effective wavelengths for driving photosynthesis.

Green light is much less effective because chlorophyll absorbs very little of it. That is also why green light can be used as a safelight when working with photosynthetic organisms, because it interferes less with photosynthesis

Importance of Oxygenic Photosynthesis

Oxygenic photosynthesis uses energy from XX to make chemical energy and releases XX (XX) as a byproduct. The overall purpose is to take XX XX and convert it into a form that cells can store and use.

During photosynthesis, light energy is first converted into the chemical energy stored in XX and XX. These molecules are then used to help build XXr. Sugar provides stored chemical energy that the plant can later break down through cellular respiration to make ATP. Sugars also provide starting materials that cells can use to build other organic molecules.

Photosynthesis is also extremely important because it produces much of the oxygen in Earth’s atmosphere, including the oxygen used during aerobic cellular respiration.

Photosynthesis can be divided into two major parts: the XX XX and the XX XX.

The light reactions occur in the XX XX. They capture light energy and convert it into XX and XX. Water is also split during the light reactions, which releases XX into the atmosphere.

The Calvin cycle occurs in the XX. It uses the XX and XX made during the light reactions to convert XX into GXX, a small sugar molecule that can eventually be used to make glucose and other carbohydrates. After ATP and NADPH are used, XX, XX , and XX return to the light reactions to be reused.

The main sequence is:

Sunlight → ATP + NADPH → Calvin cycle → sugar

and:

H₂O is split during the light reactions → O₂ is released

The big idea is that photosynthesis captures XX XX , stores that energy in XX, and produces the oxygen needed for aerobic life

Importance of Oxygenic Photosynthesis

Oxygenic photosynthesis uses energy from sunlight to make chemical energy and releases oxygen (O₂) as a byproduct. The overall purpose is to take solar energy and convert it into a form that cells can store and use.

During photosynthesis, light energy is first converted into the chemical energy stored in ATP and NADPH. These molecules are then used to help build sugar. Sugar provides stored chemical energy that the plant can later break down through cellular respiration to make ATP. Sugars also provide starting materials that cells can use to build other organic molecules.

Photosynthesis is also extremely important because it produces much of the oxygen in Earth’s atmosphere, including the oxygen used during aerobic cellular respiration.

Photosynthesis can be divided into two major parts: the light reactions and the Calvin cycle.

The light reactions occur in the thylakoid membranes. They capture light energy and convert it into ATP and NADPH. Water is also split during the light reactions, which releases O₂ into the atmosphere.

The Calvin cycle occurs in the stroma. It uses the ATP and NADPH made during the light reactions to convert CO₂ into G3P, a small sugar molecule that can eventually be used to make glucose and other carbohydrates. After ATP and NADPH are used, ADP, phosphate, and NADP⁺ return to the light reactions to be reused.

The main sequence is:

Sunlight → ATP + NADPH → Calvin cycle → sugar

and:

H₂O is split during the light reactions → O₂ is released

The big idea is that photosynthesis captures solar energy, stores that energy in sugar, and produces the oxygen needed for aerobic life

Photosynthesis:

Photosynthesis is the process that converts solar energy into XX XX . Plants take CO₂ from the air and water from the roots, then use light energy to build XX.

The overall reaction is:

Energy + 6 CO₂ + 6 H₂O → C₆H₁₂O₆ + 6 O₂

During this process, the hydrogens from XX are gradually transferred over to XX XX.

CO₂ is XX because it gains hydrogen and electrons. Those carbons are eventually used to build XX.

Water is XX because it loses hydrogen and electrons. Water is ultimately converted into O₂ gas, which is released.

So the redox relationship is:

CO₂ → XX→ XX

H₂O → XX → XX

Light Reactions

The light reactions convert XX XX into:

XX and XX

These reactions provide the chemical energy and reducing power needed to make sugar.

Calvin Cycle

The Calvin cycle uses:

XX+ XX+ XX

to build sugar.

So the light reactions make the XX and XX, and the Calvin cycle uses them to reduce CO₂ and make carbohydrate.

Alternative Carbon Fixation

Some plants use alternative ways of fixing carbon, including:

XX photosynthesis

and

XX photosynthesis

These mechanisms are especially useful in hot, dry, arid climates.

Big Picture

The whole process is basically:

Light energy → XX + XX → CO₂ XX → XX made

At the same time:

H₂O oxidized → XX released

The key exam idea is:

Photosynthesis: XX is reduced, XX is oxidized

Photosynthesis:

Photosynthesis is the process that converts solar energy into chemical energy. Plants take CO₂ from the air and water from the roots, then use light energy to build sugar.

The overall reaction is:

Energy + 6 CO₂ + 6 H₂O → C₆H₁₂O₆ + 6 O₂

During this process, the hydrogens from water are gradually transferred over to carbon dioxide.

CO₂ is reduced because it gains hydrogen and electrons. Those carbons are eventually used to build sugar.

Water is oxidized because it loses hydrogen and electrons. Water is ultimately converted into O₂ gas, which is released.

So the redox relationship is:

CO₂ → reduced → sugar

H₂O → oxidized → O₂

Light Reactions

The light reactions convert solar energy into:

ATP and NADPH

These reactions provide the chemical energy and reducing power needed to make sugar.

Calvin Cycle

The Calvin cycle uses:

CO₂ + ATP + NADPH

to build sugar.

So the light reactions make the ATP and NADPH, and the Calvin cycle uses them to reduce CO₂ and make carbohydrate.

Alternative Carbon Fixation

Some plants use alternative ways of fixing carbon, including:

C₄ photosynthesis

and

CAM photosynthesis

These mechanisms are especially useful in hot, dry, arid climates.

Big Picture

The whole process is basically:

Light energy → ATP + NADPH → CO₂ reduced → sugar made

At the same time:

H₂O oxidized → O₂ released

The key exam idea is:

Photosynthesis: CO₂ is reduced, H₂O is oxidized

Starting Photosynthesis: The Light Reactions

Photosynthesis has two major parts: the XX XX and the XX XX. The light reactions happen first and their main purpose is to make XX and XX, which are then used by the Calvin cycle.

The light reactions use XX XX and XX. Water is split in a process called XX. During photolysis, water is separated into XX, XX ions, and XX. The oxygen is released as XX gas, while the hydrogen ions and electrons are used in the light reactions to help produce XX and XX.

NADPH acts as an XX XX. It carries high-energy electrons and hydrogen from the light reactions to the XX XX. ATP provides the energy that the Calvin cycle needs.

The Calvin cycle uses the XX and XX made during the light reactions to fix CO₂ from the air and build sugar. So the light reactions make the energy-carrying molecules, and the Calvin cycle uses them to make carbohydrate.

The overall relationship is:

Light reactions: XX + XX → XX + XX + XX

Calvin cycle: XX + XX + XX→ XX

The oxygen released during photosynthesis comes from XX, not from CO₂.

The big idea is that plants take CO₂ from the air and water from the soil, use light energy to make ATP and NADPH, and then use those molecules to build sugar. That is what the phrase “life is woven out of air by light” is describing

Starting Photosynthesis: The Light Reactions

Photosynthesis has two major parts: the light reactions and the Calvin cycle. The light reactions happen first and their main purpose is to make ATP and NADPH, which are then used by the Calvin cycle.

The light reactions use light energy and water. Water is split in a process called photolysis. During photolysis, water is separated into oxygen, hydrogen ions, and electrons. The oxygen is released as O₂ gas, while the hydrogen ions and electrons are used in the light reactions to help produce ATP and NADPH.

NADPH acts as an electron carrier. It carries high-energy electrons and hydrogen from the light reactions to the Calvin cycle. ATP provides the energy that the Calvin cycle needs.

The Calvin cycle uses the ATP and NADPH made during the light reactions to fix CO₂ from the air and build sugar. So the light reactions make the energy-carrying molecules, and the Calvin cycle uses them to make carbohydrate.

The overall relationship is:

Light reactions: light + H₂O → ATP + NADPH + O₂

Calvin cycle: CO₂ + ATP + NADPH → sugar

The oxygen released during photosynthesis comes from water, not from CO₂.

The big idea is that plants take CO₂ from the air and water from the soil, use light energy to make ATP and NADPH, and then use those molecules to build sugar. That is what the phrase “life is woven out of air by light” is describing

How Does a Photosystem Harvest Light?

A photosystem is a cluster of XX and XX XX embedded in the XX XX. Its job is to capture XX XX and use that energy to XX electrons so the light reactions of photosynthesis can begin.

The first part of a photosystem is theXX XX XX. This contains many pigment molecules, including chlorophyll a, chlorophyll b, and carotenoids, held in place by proteins. When sunlight strikes these pigments, their electrons become XX. The energy is passed from pigment to pigment through the complex until it reaches the XX XX.

The reaction center contains XX and a special pair of XX XX XX. In Photosystem II, this special chlorophyll pair is called XX because it absorbs light best at about 680 nm. Photosystem I has a similar reaction center called P700, which absorbs best around 700 nm.

When enough light energy reaches the reaction-center chlorophyll, its electrons become highly XX. These excited electrons are then transferred to the XX XX XX The primary electron acceptor takes the electrons away from XX and passes them into a series of XX XX.

At this point, the reaction-center chlorophyll has lost XX, so it has been XX.Those electrons will move through the photosynthetic electron transport pathway and help drive the rest of the light reactions.

So the basic sequence is:

Light → light-harvesting pigments → reaction-center chlorophyll → primary electron acceptor → electron transport chain

The important parts of a photosystem are:

Light-harvesting complex → captures XX and transfers the energy XX

• Reaction center → contains the special XX a pair

• Primary electron acceptor → accepts the XX XX and starts XX XX

The main idea is that pigments do not all send their own electrons down the pathway. Most of them capture light energy and transfer that energy toward the reaction center. The reaction-center chlorophyll is the one that ultimately gives up the excited electrons to the primary electron acceptor

How Does a Photosystem Harvest Light?

A photosystem is a cluster of proteins and pigment molecules embedded in the thylakoid membrane. Its job is to capture light energy and use that energy to excite electrons so the light reactions of photosynthesis can begin.

The first part of a photosystem is the light-harvesting complex. This contains many pigment molecules, including chlorophyll a, chlorophyll b, and carotenoids, held in place by proteins. When sunlight strikes these pigments, their electrons become excited. The energy is passed from pigment to pigment through the complex until it reaches the reaction center.

The reaction center contains proteins and a special pair of chlorophyll a molecules. In Photosystem II, this special chlorophyll pair is called P680 because it absorbs light best at about 680 nm. Photosystem I has a similar reaction center called P700, which absorbs best around 700 nm.

When enough light energy reaches the reaction-center chlorophyll, its electrons become highly excited. These excited electrons are then transferred to the primary electron acceptor. The primary electron acceptor takes the electrons away from chlorophyll and passes them into a series of electron carriers.

At this point, the reaction-center chlorophyll has lost electrons, so it has been oxidized. Those electrons will move through the photosynthetic electron transport pathway and help drive the rest of the light reactions.

So the basic sequence is:

Light → light-harvesting pigments → reaction-center chlorophyll → primary electron acceptor → electron transport chain

The important parts of a photosystem are:

• Light-harvesting complex → captures light and transfers the energy inward

• Reaction center → contains the special chlorophyll a pair

• Primary electron acceptor → accepts the excited electrons and starts electron transport

The main idea is that pigments do not all send their own electrons down the pathway. Most of them capture light energy and transfer that energy toward the reaction center. The reaction-center chlorophyll is the one that ultimately gives up the excited electrons to the primary electron acceptor

Photosystem II: Photolysis (Water Splitting)

Photosystem II (PSII) is located in the XX XX. When light excites the reaction-center chlorophyll (P680), it loses electrons to the XX XX XX.These electrons must be replaced.

Photolysis is the XX of water to replace the electrons lost by PSII.

Reaction:

2H2O→4 H+ 4e-+O2

• Electrons (e⁻): Replace the XX lost by XX

• Hydrogen ions (H⁺): Build up in the XX XX, helping create the proton gradient used to make XX

• Oxygen (O₂): Released into the atmosphere as a XX

Important: Water is oxidized because it loses electrons.

Main sequence to memorize:

XX excites PSII → PSII loses XX → Water XX → Electrons replace those lost → H⁺ contributes to XX production → XX is released.

Photosystem II: Photolysis (Water Splitting)

Photosystem II (PSII) is located in the thylakoid membrane. When light excites the reaction-center chlorophyll (P680), it loses electrons to the primary electron acceptor. These electrons must be replaced.

Photolysis is the splitting of water to replace the electrons lost by PSII.

Reaction:

\[ 2H_2O \rightarrow 4H^+ + 4e^- + O_2 \]

• Electrons (e⁻): Replace the electrons lost by PSII.

• Hydrogen ions (H⁺): Build up in the thylakoid lumen, helping create the proton gradient used to make ATP.

• Oxygen (O₂): Released into the atmosphere as a byproduct.

Important: Water is oxidized because it loses electrons.

Main sequence to memorize:

Light excites PSII → PSII loses electrons → Water splits → Electrons replace those lost → H⁺ contributes to ATP production → O₂ is released.

Light Reactions: Noncyclic Electron Flow

Noncyclic electron flow uses both XX XX (PSII) and XX (PSI) to produce XX and XX for the Calvin cycle.

1. Photosystem II (P680)

Light excites electrons in XX, causing them to move to the primary XX XX. Water is split through XX to replace the lost electrons.

2H₂O → 4H⁺ + 4e⁻ + O₂

• Electrons replace those lost by PSII.

• H⁺ contributes to the proton gradient in the thylakoid lumen.

• O₂ is released as a byproduct.

2. Electron Transport Chain (ETC)

Electrons from PSII travel through the ETC and xx xx.This energy helps pump X into the XX XX, creating a XX XX

H⁺ then flows through XX XX, producing XX.

3. Photosystem I (P700)

Electrons reach XX and receive a second boost of XX from XX

The excited electrons pass to XX (Fd) and then NADP⁺ XX, which helps convert XX into XX

4. Why is it called noncyclic?

Electrons do not return to where they XX. They begin in XX and eventually end up in XX

Main pathway:

H₂O → PSII (P680) → ETC → PSI (P700) → Ferredoxin → NADP⁺ reductase → NADPH

Cyclic vs. Noncyclic Electron Flow

• Noncyclic flow: Uses XX and XX To produce XX, XX, and XX.

• Cyclic flow: Uses XX to produce additional XX only. No XX orXX is produced.

Important: Cyclic flow occurs when the plant needs more XX relative to XX

ATP and NADPH to Remember

The Calvin cycle requires:

• Per net G3P: X ATP + X NADPH

• Per glucose: XX ATP + XX NADPH

These are the amounts the Calvin cycle uses, not fixed amounts produced by every round of light reactions

Light Reactions: Noncyclic Electron Flow

Noncyclic electron flow uses both Photosystem II (PSII) and Photosystem I (PSI) to produce ATP and NADPH for the Calvin cycle.

1. Photosystem II (P680)

Light excites electrons in PSII, causing them to move to the primary electron acceptor. Water is split through photolysis to replace the lost electrons.

\[ 2H_2O \rightarrow 4H^+ + 4e^- + O_2 \]

• Electrons replace those lost by PSII.

• H⁺ contributes to the proton gradient in the thylakoid lumen.

• O₂ is released as a byproduct.

2. Electron Transport Chain (ETC)

Electrons from PSII travel through the ETC and lose energy. This energy helps pump H⁺ into the thylakoid lumen, creating a proton gradient.

H⁺ then flows through ATP synthase, producing ATP.

3. Photosystem I (P700)

Electrons reach PSI and receive a second boost of energy from sunlight.

The excited electrons pass to ferredoxin (Fd) and then NADP⁺ reductase, which helps convert NADP⁺ into NADPH.

4. Why is it called noncyclic?

Electrons do not return to where they started. They begin in water and eventually end up in NADPH.

Main pathway:

H₂O → PSII (P680) → ETC → PSI (P700) → Ferredoxin → NADP⁺ reductase → NADPH

Cyclic vs. Noncyclic Electron Flow

• Noncyclic flow: Uses PSII and PSI to produce ATP, NADPH, and O₂.

• Cyclic flow: Uses PSI to produce additional ATP only. No NADPH or O₂ is produced.

Important: Cyclic flow occurs when the plant needs more ATP relative to NADPH.

ATP and NADPH to Remember

The Calvin cycle requires:

• Per net G3P: 9 ATP + 6 NADPH

• Per glucose: 18 ATP + 12 NADPH

These are the amounts the Calvin cycle uses, not fixed amounts produced by every round of light reactions.

Light Reactions: Noncyclic and Cyclic Electron Flow

The light reactions happen in the XX XX. Light strikes Photosystem XX (PSII, P680), which leads to the splitting of XX. Water is split into XX, XX XX, and XX. The electrons replace the electrons lost from XX, while the X ions begin building up inside the XX XX.

The thylakoid lumen becomes very XX because so many X ions accumulate there. The XX has a lower concentration of H⁺, so a strong proton gradient develops across the XX XX. This is similar to the proton gradient created in XX during cellular respiration.

The electrons from PSII move through anXX XX XX. As they move through the cytochromes and other carriers, energy is used to move additional X ions into the XX XX. The electrons eventually reach XX XX (PSI, P700), where light gives them another boost of XX.

From PSI, the high-energy electrons are passed to XX (Fd) and then to XX XX. NADP⁺ reductase uses the XX and X to reduce XX into XX. NADPH then carries those electrons and hydrogen to the XX XX, where they will be used to help make sugar.

The H⁺ ions that have accumulated in the thylakoid lumen cannot freely cross the XX. Their main route back into the stroma is through XX XX. As H⁺ flows through ATP synthase, the enzyme spins and uses that energy to combine XX + XX into XX.

So in noncyclic electron flow, the overall sequence is:

XX →XX→ XX XX XX → XX →XX → NADP⁺ reductase → XX

At the same time:

XX → XX → XX is made

The light reactions therefore produce both XX and XX.

The reason cyclic electron flow is needed is that the Calvin cycle requires more XX than NADPH. If electrons always go from ferredoxin directly to NADP⁺ reductase, the plant does not make quite enough ATP.

In cyclic electron flow, some electrons from ferredoxin are redirected back to the cytochrome/electron transport chain instead of going straight to XX XX. As those electrons pass through the transport chain again, more H⁺ ions are moved into the thylakoid lumen.

Those extra H⁺ ions then flow through XX XX, producing additional ATP. The electrons eventually return to XX, so they are being recycled rather than ending up in XX.

That means:

Noncyclic electron flow → ATP + NADPH + O₂

Cyclic electron flow → extra ATP only

Cyclic flow does not produce extra NADPH and does not split additional water directly. Its main purpose is to increase XX XX

According to this slide, producing one G3P requires about 9 ATP and 6 NADPH, while producing one glucose requires about 18 ATP and 12 NADPH. Because that ratio is more ATP than NADPH, cyclic electron flow helps provide the extra ATP needed.

A useful way to remember the whole process is:

PSII splits water and supplies electrons

ETC builds the H⁺ gradient

PSI re-energizes electrons

NADP⁺ reductase makes NADPH

ATP synthase makes ATP

Cyclic flow gives extra ATP when needed

Light Reactions: Noncyclic and Cyclic Electron Flow

The light reactions happen in the thylakoid membrane. Light strikes Photosystem II (PSII, P680), which leads to the splitting of water. Water is split into electrons, H⁺ ions, and O₂. The electrons replace the electrons lost from PSII, while the H⁺ ions begin building up inside the thylakoid lumen.

The thylakoid lumen becomes very acidic because so many H⁺ ions accumulate there. The stroma has a lower concentration of H⁺, so a strong proton gradient develops across the thylakoid membrane. This is similar to the proton gradient created in mitochondria during cellular respiration.

The electrons from PSII move through an electron transport chain. As they move through the cytochromes and other carriers, energy is used to move additional H⁺ ions into the thylakoid lumen. The electrons eventually reach Photosystem I (PSI, P700), where light gives them another boost of energy.

From PSI, the high-energy electrons are passed to ferredoxin (Fd) and then to NADP⁺ reductase. NADP⁺ reductase uses the electrons and H⁺ to reduce NADP⁺ into NADPH. NADPH then carries those electrons and hydrogen to the Calvin cycle, where they will be used to help make sugar.

The H⁺ ions that have accumulated in the thylakoid lumen cannot freely cross the membrane. Their main route back into the stroma is through ATP synthase. As H⁺ flows through ATP synthase, the enzyme spins and uses that energy to combine ADP + Pi into ATP.

So in noncyclic electron flow, the overall sequence is:

H₂O → PSII → electron transport chain → PSI → ferredoxin → NADP⁺ reductase → NADPH

At the same time:

H⁺ accumulates in lumen → H⁺ flows through ATP synthase → ATP is made

The light reactions therefore produce both ATP and NADPH.

The reason cyclic electron flow is needed is that the Calvin cycle requires more ATP than NADPH. If electrons always go from ferredoxin directly to NADP⁺ reductase, the plant does not make quite enough ATP.

In cyclic electron flow, some electrons from ferredoxin are redirected back to the cytochrome/electron transport chain instead of going straight to NADP⁺ reductase. As those electrons pass through the transport chain again, more H⁺ ions are moved into the thylakoid lumen.

Those extra H⁺ ions then flow through ATP synthase, producing additional ATP. The electrons eventually return to PSI, so they are being recycled rather than ending up in NADPH.

That means:

Noncyclic electron flow → ATP + NADPH + O₂

Cyclic electron flow → extra ATP only

Cyclic flow does not produce extra NADPH and does not split additional water directly. Its main purpose is to increase ATP production.

According to this slide, producing one G3P requires about 9 ATP and 6 NADPH, while producing one glucose requires about 18 ATP and 12 NADPH. Because that ratio is more ATP than NADPH, cyclic electron flow helps provide the extra ATP needed.

A useful way to remember the whole process is:

PSII splits water and supplies electrons

ETC builds the H⁺ gradient

PSI re-energizes electrons

NADP⁺ reductase makes NADPH

ATP synthase makes ATP

Cyclic flow gives extra ATP when needed

Comparison of Chemiosmosis in Mitochondria and Chloroplasts

Chemiosmosis is the process of using an XX (proton) gradient to produce XX through XX XX.

Similarities:

Both mitochondria and chloroplasts use an XX XX XX(ETC) to create an XX XX across a membrane. H⁺ flows from an area of XX XX to low concentration through ATP synthase, which produces ATP from ADP + Pi.

Mitochondria:

• ETC is located in the XX XX XX

• Electrons come from XX andXX

• H⁺ accumulates in the XX XX

• H⁺ flows through XX XX into the matrix.

• XX is produced for cellular work.

• XX is the final electron acceptor, forming water.

Chloroplasts:

• ETC is located in the XX XX

• XX XX drives electron transport, and electrons come from XX

• H⁺ accumulates in the XX XX

• H⁺ flows through ATP synthase into the XX.

• XX and XX are used in the Calvin cycle to build sugars.

• OXX is released as a byproduct of water splitting.

Main pathways to memorize:

Mitochondria: H⁺ accumulates in XX XX → flows into XX→ XX produced.

Chloroplasts: H⁺ accumulates in XX XX→ flows into XX → XX produced.

Both: ETC → H⁺ gradient → ATP synthase → ATP

Comparison of Chemiosmosis in Mitochondria and Chloroplasts

Chemiosmosis is the process of using an H⁺ (proton) gradient to produce ATP through ATP synthase.

Similarities:

Both mitochondria and chloroplasts use an electron transport chain (ETC) to create an H⁺ gradient across a membrane. H⁺ flows from an area of high concentration to low concentration through ATP synthase, which produces ATP from ADP + Pi.

Mitochondria:

• ETC is located in the inner mitochondrial membrane.

• Electrons come from NADH and FADH₂.

• H⁺ accumulates in the intermembrane space.

• H⁺ flows through ATP synthase into the matrix.

• ATP is produced for cellular work.

• Oxygen is the final electron acceptor, forming water.

Chloroplasts:

• ETC is located in the thylakoid membrane.

• Light energy drives electron transport, and electrons come from water.

• H⁺ accumulates in the thylakoid lumen.

• H⁺ flows through ATP synthase into the stroma.

• ATP and NADPH are used in the Calvin cycle to build sugars.

• Oxygen is released as a byproduct of water splitting.

Main pathways to memorize:

Mitochondria: H⁺ accumulates in intermembrane space → flows into matrix → ATP produced.

Chloroplasts: H⁺ accumulates in thylakoid lumen → flows into stroma → ATP produced.

Both: ETC → H⁺ gradient → ATP synthase → ATP

Calvin Cycle: Turning CO₂ Into Sugar

The Calvin cycle occurs in the XX of the chloroplast. It uses XX and XX from the light reactions to convert XX into XX, which can be used to build glucose.

The Calvin cycle has 3 phases:

1. XX XX

2. XX

3. XX

4. 
   

The cycle begins with X XX molecules (ribulose-1,5-bisphosphate). Each RuBP has X carbons.

3 RuBP × 5 carbons = 15 carbons

Then 3 CO₂ molecules enter, adding X more carbons.

The enzyme XX attaches CO₂ to RuBP, forming three unstable 6-carbon molecules that immediately split into 6 molecules of 3-PGA.

3-PGA (3-phosphoglycerate) is a 3-carbon organic acid.

2. Reduction

• X ATP provide energy to XX the 6 molecules of 3-PGA.

• X NADPH provide electrons to XX The molecules.

• This produces X G3P molecules, each containing 3 carbons.

• X G3P leaves the cycle and can be used to build sugars.

• X G3P remain for regeneration.

3. Regeneration

The remaining X G3P molecules contain XX carbons.

These are rearranged using 3 ATP to regenerate 3 RuBP molecules, allowing the cycle to continue.

X CO₂ + X ATP + X NADPH → X net G3P

X CO₂ + XX ATP + XX NADPH → enough G3P to make X glucose

Important Numbers to Memorize

	1 net G3P	Enough G3P for 1 glucose
CO₂	3	6
ATP	9	18
NADPH	6	12
Net G3P	1	2

Carbon Tracking

3 RuBP (15 C) + 3 CO₂ (3 C) → 6 3-PGA (18 C) → 6 G3P (18 C)

Then:

• 
  

• 
  

• 1 G3P (3 C) leaves the cycle.

• 5 G3P (15 C) regenerate 3 RuBP.

Main idea: Carbon fixation adds CO₂, reduction produces G3P using ATP and NADPH, and regeneration restores RuBP using ATP.

X CO₂ + X ATP + XNADPH → X net G3P

X CO₂ + XXATP + XX NADPH → enough G3P to make 1 glucose

Calvin Cycle: Turning CO₂ Into Sugar

The Calvin cycle occurs in the stroma of the chloroplast. It uses ATP and NADPH from the light reactions to convert CO₂ into G3P, which can be used to build glucose.

The Calvin cycle has 3 phases:

1. Carbon fixation

2. Reduction

3. Regeneration

1. Carbon Fixation

The cycle begins with 3 RuBP molecules (ribulose-1,5-bisphosphate). Each RuBP has 5 carbons.

3 RuBP × 5 carbons = 15 carbons

Then 3 CO₂ molecules enter, adding 3 more carbons.

The enzyme Rubisco attaches CO₂ to RuBP, forming three unstable 6-carbon molecules that immediately split into 6 molecules of 3-PGA.

3-PGA (3-phosphoglycerate) is a 3-carbon organic acid.

2. Reduction

• 6 ATP provide energy to phosphorylate the 6 molecules of 3-PGA.

• 6 NADPH provide electrons to reduce the molecules.

• This produces 6 G3P molecules, each containing 3 carbons.

• 1 G3P leaves the cycle and can be used to build sugars.

• 5 G3P remain for regeneration.

3. Regeneration

The remaining 5 G3P molecules contain 15 carbons.

These are rearranged using 3 ATP to regenerate 3 RuBP molecules, allowing the cycle to continue.

Important Numbers to Memorize

3 CO₂ + 9 ATP + 6 NADPH → 1 net G3P

6 CO₂ + 18 ATP + 12 NADPH → enough G3P to make 1 glucose

	1 net G3P	Enough G3P for 1 glucose
CO₂	3	6
ATP	9	18
NADPH	6	12
Net G3P	1	2

Carbon Tracking

3 RuBP (15 C) + 3 CO₂ (3 C) → 6 3-PGA (18 C) → 6 G3P (18 C)

Then:

• 1 G3P (3 C) leaves the cycle.

• 5 G3P (15 C) regenerate 3 RuBP

Difference Between C₃, C₄, and CAM Photosynthesis

C₃ photosynthesis is the standard pathway. XX enters the leaf and is fixed directly by XX in the Calvin cycle. The first stable product is a X-carbon compound, which is why it is called C₃ photosynthesis.

C₄ photosynthesis separates XX XX and the XX XX by location. This is called spatial separation. CO₂ first enters a XX cell, where XX XX fixes it into a X-carbon organic acid. That 4-carbon compound then moves to a XX XX cell, where CO₂ is released and enters the Calvin cycle. This helps keep CO₂ concentrated around Rubisco and reduces problems with oxygen. Examples include corn and sugarcane.

So for C₄:

Mesophyll cell → CO₂ fixed into 4-carbon acid

then

Bundle-sheath cell → CO₂ released → Calvin cycle

CAM photosynthesis also initially fixes CO₂ into a X-carbon organic acid, but it separates the steps by XX instead of by different cell types. This is called XX XX.

In CAM plants, such as cacti and pineapple, CO₂ enters mainly at XX. The CO₂ is temporarily fixed into an organic acid and stored. During the XX, when sunlight is available, that organic acid releases CO₂, and the CO₂ enters the Calvin cycle to make sugar.

So for CAM:

Night → XX enters and is stored in organic acid

Day → XX is released → Calvin cycle

The easiest comparison is:

C₃: no XX XX; Rubisco fixes CO₂ directly

C₄: XX XX
XX fixation and Calvin cycle happen in XX XX

CAM: XX XX
CO₂ fixation and Calvin cycle happen at XX XX

C₄ and CAM are both useful adaptations for plants living in hot, dry conditions, because they help reduce water loss and improve carbon fixation efficiency

Difference Between C₃, C₄, and CAM Photosynthesis

C₃ photosynthesis is the standard pathway. CO₂ enters the leaf and is fixed directly by Rubisco in the Calvin cycle. The first stable product is a 3-carbon compound, which is why it is called C₃ photosynthesis.

C₄ photosynthesis separates carbon fixation and the Calvin cycle by location. This is called spatial separation. CO₂ first enters a mesophyll cell, where PEP carboxylase fixes it into a 4-carbon organic acid. That 4-carbon compound then moves to a bundle-sheath cell, where CO₂ is released and enters the Calvin cycle. This helps keep CO₂ concentrated around Rubisco and reduces problems with oxygen. Examples include corn and sugarcane.

So for C₄:

Mesophyll cell → CO₂ fixed into 4-carbon acid

then

Bundle-sheath cell → CO₂ released → Calvin cycle

CAM photosynthesis also initially fixes CO₂ into a 4-carbon organic acid, but it separates the steps by time instead of by different cell types. This is called temporal separation.

In CAM plants, such as cacti and pineapple, CO₂ enters mainly at night. The CO₂ is temporarily fixed into an organic acid and stored. During the day, when sunlight is available, that organic acid releases CO₂, and the CO₂ enters the Calvin cycle to make sugar.

So for CAM:

Night → CO₂ enters and is stored in organic acid

Day → CO₂ is released → Calvin cycle

The easiest comparison is:

C₃: no special separation; Rubisco fixes CO₂ directly

C₄: spatial separation
CO₂ fixation and Calvin cycle happen in different cells

CAM: temporal separation
CO₂ fixation and Calvin cycle happen at different times

C₄ and CAM are both useful adaptations for plants living in hot, dry conditions, because they help reduce water loss and improve carbon fixation efficiency

All oxygen you breathe comes from water.

All oxygen you breathe comes from water.